The West supported or instigated (which was it?) a coup against the Iranian government in order to install the authoritarian Shah. When he was overthrown in a revolution in 1979 and replaced with a relatively authoritarian theocratic regime the West/US didn’t like it. It didn’t like it because the revolution was done on behalf of some subset of Iranians; it was no longer a Western puppet. That’s why the US is anti-Iran. End of.