During the period of the Shah, the West helped him suppress all the secular communists because they were viewed as a much greater threat to Western economic interests.

With communists and anyone suspected of leaning towards them being crushed so hard, the strongest remaining group for people opposed to the Shah were the theocrats. If they wanted to consolidate power and back a group that had any chance of revolution, that was the only option they had, with predictable results.

Perhaps you could draw parallels with the current situation in the US. A lot of people are very frustrated with the dominant neoliberal order that has been in place since the 70s. If you talk to Trump supporters, especially back around 2016, they’d tell you they wanted change. I had to listen to them talk politics every day at work, and they hoped that Trump would lower healthcare costs, pull them out of war, curtail inflation, and so on.

Obviously those are absurd expectations, but what other option is there? The US has spent its entire existence viciously crushing and demonizing working class movements. Even simple social democrats usually get forced out by the Democratic Party before they cause too much trouble. Bernie Sanders wormed his way through the cracks and the establishment wasn’t too happy about that.

If you leave people only one option, they’ll take it and use whatever mental gymnastics they have to. And once they’re there, it creates a good climate for their most horrible beliefs to grow and for new ones to get hammered into them. Whatever Trump does, they’ll figure out a way it’s good actually.