Poverty. The answer is always poverty. Evacuation is not free and never has been.
Currently sitting in the Tampa Bay area while a category 5+ hurricane comes barreling at me. I'm in flood zone D next to E so I shouldn't have any issues there. In a building that is solid brick/cinderblock construction, built like a bunker. Don't worry about me. I got water, food, and enough fat to get me through the winter as they say.
The one thing I don't have is the hundreds or thousands of dollars it would take to drive 2+ states away and get a hotel for a week. I simply don't have it. Then you have all these people in places like Missouri or Montana posting this question about why people would not evacuate. We don't have the goddamn money. It's not hubris. We SHOULD evacuate. I don't see any of the people saying this offering up a spot on their couch. We should always evacuate... somewhere other than the house of the person who thinks we should, apparently.
For me it's earthquakes and tsunamis. You can survive the sky, but when the earth and the sea turn against you there is no hope.
There are organizations helping with evac, but it's mostly folks like us - lefties, ad hoc anarchist efforts, churches, and just regular decent people. It's not enough, but, well, it's what we've got.