So for many of them, it wasn't that they didn't evacuate for being unwilling to leave. They had no way to get out. Which makes this a very different case than someone who stayed in Gulfport when they had a car to get out.
When government fails, society must pick up the slack, and failing that, individual initiative must rule the day. When individuals are allowed/have been trained to do nothing in direct contravention of self-interest then they mill about the streets of a flooded city-cum-toxic-swamp blaming everyone else for their troubles.
No car, no bus, no train, but you still have feet. Or how about people who did have cars or other means of personal transport looking after their neighbors? Once the storm hit it very quickly became every man for himself and that was way too late for many. I thought it was disgusting how the first places abandonded, apparently, were the nursing homes, with the patients still locked inside. Great commentary on human nature, don't you think? Whatever happened to forethought and planning? Not like the newspapers and television stations in New Orleans were not repeating, at least three times a day, what people should do in an emergency. The first rule of any emergency: be prepared.
The real issues here are not "what did the government (at all levels) do for the people?" but "what did the peope do for themselves?"
The answer, in any case, to both rhetorical questions is: nothing.
I know this question has been asked of you before, but pray tell, if these people were on foot, where and how far were they going to go - and what were they going to do when they got there?
The problem is not that the bridge was "closed" by the local sheriff and they couldn't hoof it to Houston or even to Gretna. If they had gotten in, they likely would have taken to looting Gretna - certainly for water (the only thing essential at this time) and likely a lot else. That means breakins, fires, mayhem, possibly violence against whoemever was there. Chaos.
The problem was, aid was not allowed in by the State and the local government had completely broken down. Looting and vandalism was rampant in NO. This would likely have occured in Gretna, and the grannies trapped in wheelchairs at the Terrordome still would not have gotten food and water from those who had managed to make it into town.
Of course, Shep and Whorealdo would have been pleased...
I live on a dead end county public road with only a few handfuls of houses on it. No stores, no gas stations. If the same terrible situation happened here, rest assured we residents would close our road, a public road, from wandering people - some portion of which might be interested in a lot more than food and water. If we saw a real need and had the means we would provide help at the head of the road. But nobody is getting in unless they are legitimate law enforcement or accompanied by them.