Those most likely to fight the mullahs- the educated upper-middle and upper classes - were allowed to emigrate. A big wave of expats came to the US in the mid-70s. Some were loyalists to the Shah (e.g. military officer families, and thus in the highest social stratum); others saw the writing on the wall and left while they could.
In essence, they could come to the US or Europe and maintain their high socioeconomic status, or risk losing everything under the mullahs.
The Shah was corrupt and not unlike Saddam Hussein in political philosophy (Baath/authoritarian). Probably many Iranians thought that the revolution would bring about change for the better.
Ok that helps me understand that somewhat - thank-you!