Okay I was wrong

It's not so amazing that there are 80,000 folks who didn't evacuate.

It seems that most of these people are too poor to have a car and have to use public transportation. So when the transit shut down they couldn't evacuate. Or they had a car and couldn't afford the gas to get out of town. Or perhaps they had no where to go or didn't have the money to stay anyplace once they did get out.

And leaving New Orleans on foot couldn't have been an attractive option.

But the city was only prepared to deal with a category 3 storm (I may be misremembering and it was cat 4) and Katrina was a category 5. And having lived through the Great Missouri Flood of 93 I can sort of understand why they wouldn't prepare the town for an event that is so unlikely to happen. That flood was what they call a 500 year event (there is only one event of that magnitude every 500 years). I haven't heard how infrequent a hurricane this severe is but the media is saying that only the 1906 San Francisco fire was worse. So it's nearly a 100 year event even if not the same type.