Because it has no way of achieving its objectives.
I doubt the revolutionaries sympathizers within Iran liked their children being murdered or infrastructure getting destroyed. All the US has done is a repeat of the same thing they've done for half a century: start a war and immediately get more enemies within the middle east. Perhaps the only change is now the US's allies are distancing themselves faster and further than ever before.
So then what would we achieve? nuclear material is cheap (10s of billions) relative to a multi-decade occupation (single digit trillions). It's undoubtedly true that Iran would revert to it's preferred form of government, geopolitical orientation, and nuclear capability once the US left.
The lack of will to use sufficient force to win a war is fundamentally no different from not having that force in the first place. Both are equally real constraints on your ability to win the war.
Because Trump is already facing a bloodbath in the midterms and his next step is either a ground war or dropping a nuke, and both of those will ensure he not only loses the midterms but has a legitimate shot at seeing the inside of a prison cell.
Knocking off Saddam gave us ISIS. These things have a way of going sideways.
Let's see. It may be a military dictatorship using Khamenei, who may or may not even be in Iran, as a figurehead.
So it would still be a theocracy, same as before, but now also run by people who are conditioned to believe that more violence is always a solution to any problem.
