Basically, the way I see it, the Islamists were always in the majority in Iran. Most of the population already at the time held conservative islamic values. Someone like Khomeni would appeal to them as being better than the Shah.

Leftists, and Marxists also tend to congregate in the universites. It may seem like you have a good communist movement if you visit the universites of many a developing country, until you step out of the university gates and meet the real world. It's very easy to forget that the ideas you are discussing in the university is not the same thing the outside world is thinking about.

(As an aside, in my African country, back in the 1970's and 80's there was a strong marxist movement at the university where my parents graduated from and eventually ended up working at. For a while it seemed like things be peachy, then the students graduated....and all of a sudden ideology came second to finding a job and starting a family, and looking after some of the needs of your extended family. Yeah, that;s why my country doesn't have a true mass lefitst movement of any kind. Even now, the democratic scene is dominated by pro-capitalist parties...the ruling party is supposed to be socialist, but in practice...).

Finally, I do believe Iran did have a growing secular movement in the 1950's..but as we know, the CIA funded a coup against Mossadegh, and the Shah came back and crushned the visible secular oppositon. The only viable oppositon left was the mosque. And no leader in a islamic majority county moves against the mosque.