This is a bit of a tricky question to answer due to how it is formulated. First, the spread of Zoroastrianism is not well understood, since it occured in what amounts to pre-history as far as the study of it is concerned. However, a few points are accepted by most scholars.

Zoroaster lived probably in what is today Uzbekistan or Kazakhstan around 1300 BC (give or take a century or two), in a pastoral society. West Iranian peoples (Medes, Persians, etc) migrated into the Plateau perhaps in the 700's BC. Gathic Avestan, the language of the Gathas (which may or may not have been Zoroaster's vernacular) pre-dates the split between West and East Iranian languages (East Iranian being exemplified by Pashto and Ossetian) and is more similar to Rgvedic Sanskrit than anything else.

What this suggests is essentially that Zoroastrianism was not "accepted" at some point, it simply arrived with the Iranian peoples who settled in the region; the idea of a long-surviving pre-Zoroastrian "Mithraism" is not today accepted by most scholars. However, everything suggests that Zoroastrianism was historically a diverse religion - unified by a peculiar moralism and the worship of Ahura Mazda (indeed, the self-referential term is generally Mazdayasni, or Mazda-worshipper), but otherwise taking various forms and interacting with local religions - we see a lot of Mesopotamian influence in deities such as Anahita, identification of Ahura Mazda with Zeus, and it is very likely that Elamite interaction shaped Persian religious practices (such as strengthening the association with Mazda and the sky).

So, the religions of the region before the arrival of Iranian peoples would probably have been a diverse collection of polytheistic practices, presumably to some degree connected to the practices of local powers, e.g. Urartian and Elamite religions. Unfortunately, I'm not super well versed in either of them.