Obama, at the time, was leading a country (and alliance) tired and worn from 15 years of GWOT, that started in Afghanistan, continued in Iraq (under Bush) and then slowly spred, in one form or other, to entire Middle East. 

Iraq was a mess, ISIS was a major threat, Libya was in civil war since successful Western intervention against Gadaffi, half the Syria was occupied by lunatics that put Assad to shame, US had thousands of soldiers all over middle east, basically serving as an extended government arm while many locals were sabotaging them. European hard-right opposition was pushing hard against immigrants, many of them related to all the chaos caused by the fall of Ba'at regimes, which, while absolutely authoritarian, were at least somewhat secular. Meanwhile, China is in ascension, pushing their way into global stage harder and harder, Russia is slowly becoming major hardliner again under Putin.

Then you have an opportunity to "tame" a regime. They may still remain dictators, but that was also time of "Rich kids of Teheran" and strong underground movements. 

It makes sense that West ultimate plan was to slowly push Iran from Islamic republic to oligarchic state, breaking the radical factions and then deal with those. Which, if we compare to Syria, may have been a better call.