Soviet Union containment policy. Same reason CIA didn't really care about the 1979 revolution (until it was too late): in their back channel talks with Khomeini, he told them that he wasn't opposed to US interests and wasn't going to let the Soviets in. That was exactly what the US wanted to hear.

The real head-scratcher is why the US refused to extradite the Shah back to Iran for trial. What a different world we'd be in right now: the hostage crisis never would've happened. Moderates would've been empowered. Iran's completely valid historical grievances would've been addressed. Who knows, maybe the Iran-Iraq war never would've happened. The US wouldn't have provided WMDs to Saddam Hussein. No Gulf War I and II.

I understand what you are saying, but we allowed Hugo Chavez to nationalize Exxon holdings in Venezuela (forming PDVSA), which was a direct U.S. interest.

Why Mosaddegh was denied what Chavez was allowed is not clear to me.