Actually, New Orleans and the other affected areas had/have quite good evacuation plans, and most of the population evacuated to safe areas. The problem is, and I'm not really sure that "problem" is the right word, is that the plan was entirely voluntary. Even after the mayor of New Orleans declared a mandatory evacuation, there was no mechanism in place to force people to leave.

There were certainly a lot of people left behind who did not have the means to go anywhere, and there were also a lot who could have gone but were too pig-headed to leave. What's the percentage of the one and the other? Who knows, and speculation is only speculation. I read earlier today about a couple of tourists (Canadian? Australian?) staying in a Bourbon Street hotel who didn't evacuate when they had the chance because the hotel staff told them something to the effect of "it's ok...we never leave for these things"...and they ended up at the Convention Center.