Simple - keep Islam from rising. In its absence, I think it's not unlikely that Central Asia becomes at least partially Zoroastrian. In this scenario, Christianity would be the top religion, then Hinduism, with both religions being considerably larger than OTL. Assuming that Hinduism is able to beat out or absorb Buddhism in Southeast Asia (possible) that Buddhism is kept from having too strong of a hold on China, and that Persia doesn't face the demographic stagnation it faced from the Mongol invasion onwards, the result is that Zoroastrianism might eke out Buddhism.
A much easier way, of course, is that Muhammad founds a Zoroastrian sect after seeing a different vision from OTL, proclaims himself Shahanshah, and his successors found an empire as vast the OTL Caliphate - with the exception that it's Zoroastrian.