Ancient Egypt

The answer is probably that they actually weren't "so ahead", at least in Near Eastern context, then the most advanced region of the world probably. They actually didn't incorporate bronze or iron to their technologies till much later than other neighbours and their might is probably just a byproduct of being a large, densely populated centralized state, what allowed pharaohs to manage huge resources that other princes of their time (except comparable but less stable kingdoms of the region) didn't have. Another reason is maybe that, unlike Mesopotamians, specially Sumerians, they used stone and not mudbrick in their constructions, allowing them to endure te test of time much better. But you do find other comparable civilizations in the other rich areas of the Near East, particularly Mesopotamia and Asia Minor - only that Egypt, more isolated and particularly needing a centralized irrigation authority, was much more stable. 

They were in the right place in the right moment while the Near East was the "center of the world", i.e. Neolithic and Chalcolithic, they had a very productive country when ruled in a centralized manner and that made them very powerful and maginficent and somehow closed to the outside (a little like China later on). They were never particularly outstanding in sailing or warfare, but they were great in architecture, mathematics or astronomy, for instance. Their geographical position protected them from most threats, as Lybians and Nubians were comparatively smaller and more backwards. Their major enemies could only came from the Near East (and partly from the Sea).

Eventually, as the center of gravity moved a little more northwards, to Asia Minor, where Bronze was becomng common and where eventually Hittites had developed the most secret technique of iron casting, they actually became a relatively isolated and technologically retarded people, specially in metalurgy, an essential war technology. The incursions of the Sea Peoples and the destruction of Minoan civilization didn't help the Egyptians either to keep along with the march of time. Yet, they were still powerful enough as to achieve a stalemate with Hittites at Kadesh and to draw the Sea Peoples back into the waves. But eventually they were conquered by war-mongering Assyrians and later by Persians, bringing them the iron technologies but ending with their independence. For then on Egypt would be ruled by foreigners, wether as a province of a foreign empire or an independent realm under a foreign dynasty... till the late 20th century.

Still, for more than two milennia they were a powerful, proud and independent kingdom, able to build many hydrological and architectural marvels that still fascinate us. Their culture was impressive indeed but hey lost pace in the military technological race.