Why is the Iran regime so obsessed with Salman Rushdie?

I'm trying to understand why the Iran regime considers Salman Rushdie such a problem that they put millions of dollars bounties on him over the years. From my vague understanding Rushdie wrote an allegorical book that might be considered criticism of Islam? Why is Iran so bothered with Rushdie but they seemingly did not put such bounties on more obvious "blasphemies" like the Jyllands cartoons etc.?