Iran has geopolitical conflict with the West, but it's a false equivalent of Saudi Arabia. Iran is a theocracy trying to control a population that growingly wants to be secular. Women in Iran go to university at higher rates than men, can drive, hold office, and vote. Life in Iran for a women is not perfect but is heaven compared to Saudi Arabia.
Iranian sponsored militias like hezbollah are in geopolitical conflicts as Iran wants more regional influence. They want regional islam. They are not trying to establish a global Shiite islamic state.
Like most things it comes down to money and power. Saudi plays ball with America economic hegemony and Iran does not. So rather than allying with the more moderate/educated nation with huge potential to be progressive, we ally with the one that is best for big business and willing to cede military control of the region to us.
The Iranian theocracy is fucked, don't get me wrong. But we really have our priorities in a mess.