The Persians took Zoroastrianism with them into space, much like American Christianity and Israeli Judaism.  It’s simply there, wherever you look.  Printed on money, built in the hearts of their colonies, heard in the music, and everything else that makes a culture.  Nobody is forced to believe anything.  Religious liberty is the bedrock of their constitution after all.  No surprise after the Jihad.  But Zoroastrianism is just part of life in the Persian worlds.