True, a majority of people still in the city when Katrina hit were the Lower Class of New Orleans, and that's pretty damn low. These people didn't have Cars, and weren't really prepared to leave what little they had to the storm, knowing that they would lose everything and not be able to buy it back. So they either stayed where they were, or took shelter in one of the (grossly undermanaged and unplanned) shelters, ie. Astrodome...
...Now BlackDove, let's say your a poorly educated, dirt broke American living in a tinny in New Orleans. Now, you're told a Storm is coming, but you don't have a car, nobody you know has a car, and you have pretty much no way to leave the city other than walking. And several hundred miles to get out of the danger zone is just a little too far for you haul your poor, uneducated, smelly ass. Now, what are you going to do...?