Why did Iran adopt Islam?

Answer 1

Iran was conquered by the Muslims during the reign of Second Rightful Caliph Hazrat Omar RAU. The interaction between the Muslims and the local Iranians resulted into the acceptance of Islam by the local people.

Answer 2

Iran did not have much choice in the matter. At the Battle of Nahavand in 642 C.E., the Arab Muslim armies led by Sa3d ibn Abi Waqqas completely defeated and overwhelmed the Persian Zoroastrian armies of the Sassanid Empire led by King Yazdagerd III and Piruz Khosrow. After the Muslim victory at that battle, the entire Sassanid Empire was subsumed by the Rashidun Caliphate. Most of the Sassanid nobility converted to Islam to stay in power in the new Islamic Empire. As a result, the Persian nobles and kings who would come to take power in the country would consistently be Muslims.