Iranian cultural notes
To get back on-topic-
I went through all the OTL reasons why the US didn't pull a Hulk-SMASH aerial-naval pimp-slap campaign 48 hours after the hostages were taken that'd leave the Iranian oil terminals so much smoking rubble along with the rest of the Iranian economy.
If the hostages were massacred- even the most hopeful CIA or State Dept analyst would have to conclude there's no sane folks to work with and initiated Operation Whirlwind (FWIW not a real op, I named it after the James Clavell novel re Iranian Revolution. Worth a read IMO.)
Nobody Western in their right mind had any hopes of invading or occupying Iran. HWG made it clear the Shah's Zero% Approval Rating was a major deal-killer. With no acceptable puppet to back and mind the store, no invasion.
As HWG made a point of stating, the Soviets wouldn't be welcomed with open arms either, due to their shenanigans pulled during WWII.
The Tudeh (Iranian Communists) had a lot of students and were so thoroughly penetrated and neutered by SAVAK they were largely a non-factor.
Same with any secular opposition groups.
MEK- (Mujahedin-e-Khalq People's Warriors) was another leftist movement that became an underground resistance/terrorist movement funded and organized by emigres AFTER the Revolution and getting purged by the Islamists in street battles in 1981.
Iran had a parliament- the Majlis, but it was weak, unrepresentative, and mostly about rubber-stamping new public works programs to fill one royal crony or another's pockets glorifying the Shah. It didn't incubate any leaders to take over when the Shah fled.
What folks in the US Embassy never got and tried like hell to stay blind to was the profound influence Shi'a Islamists had on Iranian culture and kept hoping the goodies of Western secular culture'd sway Iranians to being happy consumers to distract Iranians from SAVAK's dirty-war tactics.
In Western culture, we're all about individual choice and the nuclear family as a result of Western liberal tradition and industrialization. We think in terms of decades and what's hip right now and throw things away just b/c they're out of style.
To Iranians, that's very isolating and terrifying compared to thousands of years of Persian culture and extended family and ethnic ties. Also, while a nomenklatura of new college grads was emerging and doing much better than before, the dead weight of the Shah and all the backdoor horse-trading and backstabbing to maintain official favor meant lots of promising folks never got jobs and felt left out as the inefficient kleptocracy kept enriching the royal court and beggaring the vast majority of Iranians.
So, to sum up, all the folks left out of the Shah's inner circle and not sucking on the oil tit really weren't doing that well, got fed up, and decided socialism sucked because it was too secular, democracy sucked because it was too easy to suborn, so hitting the reset button and making it over by explicitly Islamic principles of community values was the only social foundation that made any sense to the Iranian populace who hadn't gone to college.
Many ayatollahs had been laying this groundwork for decades but Khomeni's underground samizdat campaign of covertly distributed lectures on tape over the 1970's gave the Islamist opposition an inspirational leader laying out a clear plan of how the Shah sucked and what Islam was supposed to really be about.
What's the US got to work with in that scenario?
Were we supposed to sort the wheat from the chaff in the various ayatollahs and scholars when we'd explicitly ignored religious leaders ever since Operation Ajax installed the Shah in 1953 after deposing Mossadegh?
The folks into democracy and a secular (or least officially tolerant of Jews, Baha'i, Christians, Zoroastrians, etc) society usually had too much of a tinge of socialism to be palatable. Once the CIA decided MEK was worth supporting, they were booted out of the country and engaged in terrorist acts the Iranian authorities could brand as criminal.