Arab Conquest of what we now call Persia is the main reason. The Arabs, upon conquering land, took a carrot and stick approach

Carrot: if you were a "person of the book," meaning a christian or a jew, you could keep your religion but be forced to pay an extra tax. Zorastrians were sometimes in and sometimes out of this category. When they were in, many converted to avoid the tax. Many people converted to curry favor with their new rulers as well.

Stick: if you were not a "religion of the book," the Arabs of this time were quite brutal (so was everyone else.) For example if you were a Pagan, you were in trouble. Sometimes Zorastrians were in the not a people of the book category and then they had an obvious reason to convert.

The Arab conversion of Iran was to Sunni Islam but Iran is a majority Shia state.

This happened because of Shah Ishmael 1st, a man who claimed divinity from the Shia sect of Islam. His conquest of Persia casued yet another conversion of the people of Persia but this is out of the scope of your question.

Source: Pourshariati, Parvaneh. Decline and Fall of the Sasanian Empire: the Sasanian-Parthian Confederacy and the Arab Conquest of Iran. I.B. Tauris in Association with the Iran Heritage Foundation, 2008.