Besides what the sibling comment said my brainstorming is:
- Culture
- Cost (sedans are cheaper), but income has gone up, especially in suburbs.
- More people living in urban areas with more money, but our urban areas still need cars. People who would buy a truck if they were rural, buy a SUV when suburban.
- Smaller family sizes so minivan isn't needed.
- Gas prices have plummeted so worse MPG doesn't matter as much compared to sedans.
SUVs are a relatively recent phenomenon, with some exceptions such as the Wagoneer. And we did like large cars, but switched to minivans as cars downsized, then switched to SUVs as Minivans become stigmatized.
And even if you're ambivalent about SUVs, after a while you start thinking "It'd be nice to be able to see beyond this giant block in front of me".