Why was Persia conquered by nomadic tribes?

 Many reason have been mentioned  for that question but when I think about the conquest of an ancient civilization by nomadic tribes who had just integrated under a new ideology or religion in Arabia and when I also think that Persians were somehow dissolved in the reign of that ideology despite long and somehow severe resistance, and when again emerge as a great  Persian Empire, they adopted  that new ideology (although transformed into Shia Islam)  ,  it occurs to my mind that the battle was eventually between the two ideologies: Islam and Zoroastrianism rather than between the two nations. Bearing in mind that Persians had emerged as Persian Empire with the same ideology (Zoroastrianism) after the invasion of Alexander the Great.
 If so, what were the foibles or weak points of ancient ideologies or religions such as Zoroastrianism, Manichaeism or Mithraism that eventually declined or totally disappeared in contrast with other religions such as Islam, Christianity, Hinduism or Buddhism?