Why are we in Iraq when Iran is the threat?
There are a number of plausible answers, but the most obvious one to me is that if you ask that question, you need to study Judo. That's what it is, on a very large scale.
The problem with nuclear weapons in the hands of fanatics, state sposored or otherwise, is that their belief is that they are doing the right thing. They are following God's (by whatever name known to them) will. And that no sacrifice of others or themselves is too great to bear for the privelege of doing God's work.
MAD kept us "safe" during the Cold War. All the major players knew they had too much to lose to let things get started. The Soviet Union kept a leash on the Muslim terrorists, because of the very real worry that if they did something major, that the Soviets would be blamed, possibly turning the Cold War hot, with MAD lurking in the wings. They let the terrorists play their evil games in their own sandbox, and sometimes in Europe, but no more than that, or they risked loss of Soviet (military) support.
Today what we have is (forgive the expression) a bunch of loose cannons. Direct conventional military action against Iran is not to be undertaken without very serious provocation, because of the Muslim viewpoint that combines government and religion as one.
We could attack Saddam because he ruled Iraq not for the greater glory of God, but for the greater glory of Saddam. And even then we upset a lot of Muslims anyway. We have to be careful, otherwise we might make the people who hate us not like us.
I fond it ironic that the quote from Obama about the people in rural Pennsylvania "bitterly clinging to their guns and their religion and being distrustful of people who are not like them" is an accurate discription of so many people in that part of the world called by us, the Middle East!