I think we all tend to forget that New Orleans wasn't the only area impacted - or basically destroyed - by Hurricane Katrina. The area of devestation impacted three states with "fallout" impacting over a dozen more (i.e., TX, FL). The area is approximately the size of Great Britain.
That being said, yes MANY of those left in this area after the storm could have gotten out but choose not to for whatever reason. But, MANY also stayed behind because they simply couldn't leave - the elderly, the sick, some first-responders, and yes, dare I say it, the poor. And, in New Orleans, we all know that the mayor's ineptitude prevented the evacuation of tens of thousands who could have otherwise been transported out on those now infamous school buses.
That is exactly what I said,;those who could not get out were supposed to be rescued/evacuated by local government using those now infamous school busses.