The US gov't can't support the more moderate leaders, the true power of the country lies with the mullahs and ayatollahs who continually disqualify any "moderate" from running.
They tossed us out during their Islamic "revolution" back in the late seventies. Any move made by the U.S. in the region is denounced. Any leader supported by the west is denounced.
According to news reports the people who do vote, vote for the most moderate of the hardliners who are left, or they don't bother.
Although I believe the people of Iran probably want peace,just like the citizens of the United States, the hard line clerics there continue to demonize the United States for everything. Hard to negotiate with people who already think you're the devil.