Zoroastrianism doesn’t allow converts (or at least traditionally hasn’t). That’s presumably a key factor in why it’s shrinking off the face of the planet.
Interesting tidbit... during the late 1920’s-30’s some advisors suggested to Reza Shah Pahlevi (father of the last Shah) to make Zoroastrianism/Mazdaism the state religion of Iran, abolishing islam. However it wasn’t ever pulled through, as it was probably too radical a change (though a noble idea) and the Shah preferred general secularization/westernization instead. However he inivited the Indian Zoroastrians to return to Iran... although few came. Still the time until 1979 saw a very obvious rennaissance of Zoroastrian symbolism and thought supported by the Monarchy. I am confident if Zoroastrians would accept converts millions of Iranians would espouse it. Islam is a cult forced upon a superior civilization. Everything that is claimed to be great achievements of islamic culture are in fact Persian, Hellenistic and Christian achievements taken over by the Arabs. This includes architecture, warfare, administration, science, medicine... you name it.