Just as an aside, the primary reason for the collapse of the religion was a 20-year long religious war between Zoarastrian Persia and Christian Byzantium. This was all going on while Mohammad was creating his religion and unifying the Arab penninsula under his rule.
The war between Byzantium and Persia was quite devastating and both Empires suffered body and soul during it - Persian armies made it to the gates of Constantinople, and also siezed Jerusalem, descrating many Christian shrines; in return, and in winning the war, the Byzantines invaded the Persian heartland and destroyed the central shrine of Zoaraster. Its hard to keep your religion going at a time like that - and when an exhausted Byzantium and Persia shortly had to face a rising Islam, they just couldn't do it...
Byzantium lost Syria, Israel, Egypt and Libya, while Persia was conquered in its entirety. Given the nature of Islamic conquest and given the physical and spiritual exhaustion of Persian society, they rather rapidly converted to Islam after the conquest...in fact, such supposedly Moslem things as secluded women and wearing a Burka were originally Persian social constructs which Islam took on after the conquest of Persia...Mohammad's wives went out in public unescorted and did not wear a Burka.
Yep. Zoarastrians were killing off Romans back in ancient times, too, in the name of their religion. Although a compassionate religion, it wasn't exactly a peaceful religion when its leaders saw it as a unifying tool to incite their populations to war.
Actually the Veil was worn by Christians in Byzantines as a sign of status and wealth and that's where it originated, Zoroastrians didn't wear it - at least not on Ancient Sculptures in Persepolis.