There is no mystery. A dumb bombing campaign requires air superiority, and Iran actually has reasonable anti-air defense. They're not a bunch of guys running around with AK-74s.
The US absolutely can achieve that. But it _will_ result in aircraft losses, and it requires detailed planning. Both of these are not acceptable anymore.
To help put that in perspective: Iran's GDP today is a ~7x stronger than Iraq's was (in 2003) relative to the US GDP at each time.
In other words, they can spend more on military stuff making them a tougher opponent by default. Plus multiples of population, more land-area, etc.