"Why couldn't these people at least take what they could carry and leave as quickly as possible?"
Because they do not act until being told what to do by their government masters, to be blunt. It is really hard to blame some of them, because self reliance has not been required of them for such a long time.When I refer to "some of them" I'm speaking exclusively of the people left behind in New Orleans. I know I stand to be accused of racism for statements like that, but that is not the case. It is really hard to be honest when it comes to race issues.
They were told to get out. By the government. Several days in advance.
I do not find it at all difficult to be honest about race issues. The problem arises from not being able to speak the truth for fear of severe retribution from the PC brigade, at worst, or mere unfounded [but damaging] accusations of racism, at best.