Why Alexander did not invade India?

India or Hind(ustan) has its more ancient information in the Avesta:

The Zend Avesta (Vendidad Fargard 1) also refers to the seven rivers (hapta hendu, 1.18), which are generally equated with the Sapta Sindhu (Gnoli 1985). In the Avesta this land is referred to as one of the sixteen "best regions" of the Iranian Aryans created by the Lord.

pa�cadasem asangh�mca sh�ithran�mca vahishtem fr�thveresem azem �� ahur� mazd� �� hapta he�du,.

18. The fifteenth of the good lands and countries which I, Ahura Mazda, created, was
the Seven Rivers⁴². 42. [Hum34: "the PhlT of V1.18 quotes the fragment haca ushastara hinduua auui daosha<s>tarem hindum 'from the eastern river to the western river'.]

Hapta Hendava must be more than only the 5 Panjabi rivers. It includes the two rivers of Haryana, which last was rather the sacred center of the Vedic people. The Avestan fragment clearly refers to (N)India as the land of the rivers.

The Pahlavi or Middle Persian work Bundahishn chapter 20 refers to Hindustan:

7. � the Mehrva river they call the Hendva river,�

9. The Veh river passes on in the east, goes through the land of Sind, and flows to the sea in Hindustan, and they call it there the Mehra river.

The Mehr(v)a river is the modern Mihran, one of the channels of the ancient Sarasvati river! And that river is called the Hendva river!

Description of Hindustan, from Memoirs of  Zehīr-ed-Dīn Muhammed Bābur, Emperor of Hindustan

Written by himself, in the Chaghatāi Tūrki, Translated by John Leyden, Esq., M.D.  and William Erskine, Esq.

Annotated and Revised by Sir Lucas King, C.S.I., LL.D., F.S.A. Professor of Arabic and Persian, and Lecturer in Indian History in the University of Dublin; Late of the Indian Civil Service

http://persian.packhum.org/persian/main?url=pf%3Ffile%3D03501050%26ct%3D0 :

Events of the Year 932 (18-10-1525) ii.162

�Hindustān is situated in the first, second, and third climates. No part of it is in the fourth. It is a remarkably fine country. It is quite a different world, compared with our countries. Its hills and rivers, its forests and plains, its animals and plants, its inhabitants and their languages, its winds and rains, are all of a different nature. Although the garmsīls (or hot districts), in the territory of Kābul, bear, in many respects, some resemblance to Hindustān, while in other particulars they differ, yet you have no sooner passed the river Sind than the country,* the trees, the stones, the wandering tribes, the manners and customs of the people, are all entirely those of Hindustān. The northern range of hills has been mentioned. �

The natives of Hind call these hills Sawālak-parbat. �

On these hills the snow never melts, and from some parts of Hindustān, such as Lahore, Sirhind, and Sambal, it is seen white on them all the year round. This range of hills takes the name of Hindū-kūsh, near Kābul, and runs from Kābul eastward, but inclining a little to the south. All to the south of this range is Hindustān. To the north of these hills, and of that unknown race of men whom they call Kas, lies Tibet.*
A great number of rivers take their rise in these mountains, and flow through Hindustān. To the north of Sirhind, six rivers, the Sind, the Behāt, the Chenāb, the Rāvi, the Biās, and the Satlej,* take their rise in these mountains, and all uniting with the Sind in the territory of Multān, take the common name of the Sind, which, flowing down to the west, passes through the country of Tatta, and disembogues into the sea of Omān. Besides these six rivers, there are other rivers, such as the Jumna, the Ganges, the Rahet,* the Gūmti, the Gogra, the Sirūd, the Gandak, and a number of others, that all throw themselves into the Ganges,* which, preserving its name, proceeds towards the east, and, passing through the midst of Bengal, empties itself into the Great Ocean. The sources of all these rivers are in the Sawālak. There are, however, several other rivers, such as the Chambal, the Banās, the Betwa, and the Son, which rise from ranges of hills that are within Hindustān. In these ranges, it never snows. These rivers likewise fall into the Ganges.�

Note that Hindustan and Hind are both being used, as also the river Sind is mentioned. As per Babar, Hind(ustan) rather denotes almost entire modern Pakistan upto the Hindukush and modern India beyond the Panjab!

Another socio-geographical border is by the river Sind. But he also says about Kandahār. This country lies between Hindustān and Khorasān. I.205. Which means that Baluchistan is within Hind(ustan).

How much different is this from the descriptions of India of the classical Greaco-Roman historians? 

Ancient map of South Asia by A.Vigasin, The Indian chapter (VI.56-106) of �Natural History� by Pliny the Elder

�The starting point for the Pliny�s enumeration of the Indian peoples is again the North-West frontiers of the country, the mountains of Hindukush (Hemodus), inhabited by the tribes of Isari, Cosiri and Izi (VI.64). Cosiri (Casiri) were mentioned earlier (VI.55) as the people of Central Asia, neighbouring to Phocari (Tokharians, Τοχαροι, skr. Tukhara) and Thuni (Chineses, Θιναι, skr.Cina). Casiri were regarded as �Indians� (iam Indorum), but not civilized ones, because they practiced cannibalism (humanis corporibus vessweetieur). This tribe occupied the central part of Hindukush, near the pass between India and �Scythia� � the nomads went regularly through this pass (nomades quoque Indiae vagantur huc �VI.55). Identification of Casiri/Cosiri is uncertain, but they are to be compared with Prakrit Kasira (skr. Khasira � Mahabharata, Vulgata, VI.10.66: khasirasca tukharasca; Matsya-purana 114.34, Markandeya-purana 57.34: cinascaiva tusarasca bahula bahyato narah). The tribe is to be localized near the modern Nuristan (kafiristan), where even today the nomads cross Hindukush on their way to India. �
So the tribes are enumerated in the direction of Himalayas (Imaus) � i.e. to the North-East. We can guess only, that unidentified tribe of Isari occupied the Southern part of Hindukush.�

The words Hemodus and (H)Ima(v)us are met with in the Graeco-Roman works as giving the northern boundary of India. Indian tribes are met with even beyond the Hindukush as per Pliny.

FRAGMENT I OR AN EPITOME OF MEGASTHENES. Diodorus (c. 90-27 BC) II. 35-42.

(35.) India, which is in shape quadrilateral, has its eastern as well as its western side bounded by the great sea, but on the northern side it is divided by Mount Hemodos from that part of Skythia which is inhabited by those Skythians who are called the Sakai, while the fourth or western side is bounded by the river called the Indus, ..

FRAGM. IV. Strabo (20 BC-23 CE), XV. i. 11,--p. 689.

Of the Boundaries and Extent of India.
India is bounded on the north by the extremities of Tauros, and from Ariana to the Eastern Sea by the mountains which are variously called by the natives of these regions Parapamisos, and Hemodos, and Himaos, and other names, but by the Macedonians Kaukasos. The boundary on the west is the river Indus,

Arrian 129-216 AD (based upon older Alexandrian information)

[Excerpted from Arrian, "The Indica" in Anabasis of Alexander, together with the Indica, E. J. Chinnock, tr. (London: Bohn, 1893), ch. 1-16]

1. The district west of the river Indus as far as the river Cophen is inhabited by the Astacenians and the Assacenians, Indian tribes. But they are not so tall in stature or so courageous as those who dwell east of the Indus; nor are they so swarthy as the majority of the Indians. � Among the Assacenians is Massaca, a large city, where also is the stronghold of the land of Assacia; and there is also another large city, Peucelaitis, not far from the Indus. These tribes have been settled west of the Indus as far as the Cophen.

2. Let me call the country east of the Indus India, and the people Indians. Towards the north of India lies Mount Taurus; but in this land it is no longer called Taurus. � It is called by various names in different districts; in one part it is called Parapamisus, in another Emodus, in a third Imaus, and probably it has several other names. The Macedonians who accompanied Alexander's expedition called it Caucasus. � The river Indus bounds India on the west as far as the Great Sea, into which it discharges its water by two mouths, � Thus also the river Indus forms the Delta of India, �

3. In the whole of the rest of Asia there are not so many rivers as in India. The largest are the Ganges and the Indus, from the latter of which the country takes its name.

6. India is visited by rain in the summer, especially the mountains, Parapamisus, Emodus, and the Imaic range, and from these the rivers flow swollen and muddy. In the summer also the plains of India are visited by rain, so that a great part of them are covered with pools;

(Greater) India seems to be from the Hindu Kush down into the subcontinent. Arrian proposes to call India (proper) to be from the Indus eastward. The Astacenians (Hastikayanas or Hastinayanas of Panini) and Assacenians (Ashvakayanas of Panini. In Pali they are Assakas; modern Ashkuns?) then of the NWFP and eastern Afghanistan are called Indian tribes.

They ancient Ashvayanas of Panini (preserving the older Indo-Iranian shv),  can be traced in the closeby Aspasioi or Aspasii (changing the ancient �shv- into -sp- as an Iranism; influence of Achaemenids or Iranic Yavanas from Bactria?) , the modern Asp(a)-za(t)i.

Pliny drawing upon older material already notices changes in the western border of ancient India.
FRAGM. LVI. Pliny (23-79 CE) Hist. Nat. List of the Indian Races:

Many writers, however, do not give the river Indus as the western boundary of India, but include within it four satrapies,--the Gedrosi, Arachotae, Arii, Paropamisadae, making the river Cophes its furthest limit; though others prefer to consider all these as belonging to the Arii.

We thus do encounter the borders of ancient India as 1) Hindukush in the NW and 2) Indus river in the west in the Graeco-Roman works. This western border has to do with the difference between Bhaarata(varsha) and Aaryaavarta:

Bhaarata(varsha) The Vishnu Purana has:
uttaram yatsamudrasya himādreśhcaiva dakshinam |
varsham tadbhāratam nāma bhāratī yatra santatih || ViP_2,3.1||
"The country (varsham) that lies north of the ocean and south of the snowy mountains is called Bhāratam; there dwell the descendants of Bharata."

pūrve kirātā yasyānte paścime yavanāh sthitāh // ViP_2,3.8 //
brāhmanāh kshatriyā vaiśhyā madhye śhabdrāśh ca bhāgaśhah / ViP_2,3.9A //
On the east of Bh�rata dwell the Kir�tas (the barbarians); on the west, the Yavanas; in the centre reside Brahmans, Kshatriyas, Vaiśhyas, and Śh�dras,

Again, the Vishnu Purana description of Bhaarata is in accordance with that of (Greater) India of the Greaco-Roman historians, and also partly with Babar�s Hind(ustan).

Aaryaavarta. The Baudhayana Dharma Sutra (Titus 1.2.12) quotes a Nidana verse of the Bhallavins, which states that Aryavarta is between the Sindhu in the west and there where the sun rises:

pashcaat sindhur vidharaNii suuryasya_udayanah purah |
yaavat krShThaa vidhaavanti taavad dhi brahmavarcasam  iti ||12 ||

This is in accordance with the pre-Achaemenid work Aitareya Brahmana VIII.14.3 referring to Madhyadesha within the boundaries of Vasha (Sa-vashoshiinara), Ushinara and Kuru, Pancala.

tasmaad asyaah dhruvaayaam madhyamaayaam pratiShThaayaam dishi ye keca
kurupancaalaanaam raajaanam savashoshiinaraaNaam raajyaayaiva te �bhiShicyante |

While the Vasha-Ushinaras (Shivis, Kekayas, Madras and Vrshadarbhas) are in the Saindhva area, the Kuru-Pancala are in the Sarasvata area. And this is the Middle country, which means that the (cultural and political) horizon is larger.

Aaryaavarta or Madhyadesha in the quotes above reflects India proper of Arrian.

You are jumping the gun.  Re-read my post; I mentioned in paranthases his macedonian soldiers and officers, saying they were the core of his army.  Meaning they were the elite, implying his army was no longer solely macedonians.