So, I'm about to make an oversimplification, but it's interesting to point out some of very broad parallels between Zoroastrianism and Greco-Roman paganism. The former had been the state religion of the various incarnations of the Persian Empire for a very long time, until the proselytizing Abrahamic religion of Islam gained power and the original faith lost currency. The latter had been the state religion of the Roman Empire for a very long time, until the proselytizing Abrahamic religion of Christianity gained power and the original faith lost currency.
Now obviously there are myriad differences between both Zoroastrianism and Greco-Roman paganism as well as between the Islamization of Persia and the Christianization of the various parts of the Roman Empire. However, I want to focus on one difference in particular. You see, in Rome, after Christianity firmly took root in the Mediterranean world, Greco-Roman paganism withered away soon after. Indeed, all the evidence suggests that the religion did not survive in Greece, Italy, or anywhere else for very long after the fall of the Western Roman Empire in 476. Zoroastrianism, however, has survived to the present day. Yes, its adherents number well under a million, and a great deal of them live outside Iran, but for much of the Persian history after the Islamic conquest their numbers remained robust, to the point where I've seen it posited here that, given the right conditions, a Zoroastrian restoration could have conceivably happened in Persia well until the Middle Ages.
So my question is this: why did Greco-Roman paganism essentially die out and Zoroastrianism survive?