I can't claim to fully understand the motives of those who stayed behind. My guess is that poverty had a lot to do with it, as has been mentioned above. People just don't have the means to sustain themselves outside of their homes. There's also the surprise factor. After all, how much do
you guys trust weathermen.
For me, I realize that I'm in a completely different social class. I don't have 2 jobs just to make ends meet, and being for the most part a white 26 year old male, haven't been in too many situations where the system has failed me. I have time to sit here pondering the issues and to post my thoughts on the internet. I think that goes for more than a few of us here. Therefore I can't fully understand the motives behind staying behind, but I would err on the side of "they've got good reasons even though I can't empirically understand what they are."
Could it be that the offer of rescue is received with mistrust? Also, for the poor, the question of whether to stay and defend your property, no matter how waterlogged it might be, is a more important and relevant one than many of us here make it out to be.
In short, things aren't as simple as they seem, and there are more important things to do than lay blame on those who made the potentially wrong choice of staying behind.