Because the most powerful military in the world can only do so much when ordered to execute a crap strategy by civilian leadership. Desert Storm worked because it followed the Powell Doctrine and was bounded to ejecting Iraq from Kuwait. Iraqi Freedom muddled through incoherently because the Bush and Obama administrations didn't want to understand the actual problem set they were faced with, but ultimately ended up with a reasonably stable country after way too much effort and time. Iran is what happens when you have an administration who thinks you can win a war by "just bombing things," which is ironically the stereotype most of the internet has about any kind of military action.