For similar reasons that the USA is the great pariah in Cuba. In both cases the USA was a primary supporter of a repressive totalitarian government kicked out by a popular revolution. It's just that in Cuba the revolution was communist while in Iran it had a strongly religious dimension. Other than the Iranian's support of the Palestinian cause, there's really no particular reason for them to be a great enemy of the west, except that we totally screwed them over repeatedly for a few generations.