You got the timeline wrong there. The US installed the Shah in the 50s, after destroying Iran’s democratic leftist government. Then in the 70s, the Iranian people rose up and removed the Shah. Almost immediately, though, the revolution was hijacked by fundamentalist clerics.

When the revolutionary government was put under pressure by an invasion from Saddam Hussein on behalf of George Bush Snr, which killed millions, that allowed the Revolutionary Guard to consolidate power, and become the brutally repressive government we see today.