Bush made them do it, c'mon man, everybody knows that.
But seriously, this is kind of old news, everything post Katrina was fucked from the bottom up, which of course was the problem.
As someone who has studied incident command, I can tell you that the local, on scene authorities (Led by Nagen) completely fucked the goat there in the first critical days following the storm.
These responses, no matter how large they eventually become, are run from the bottom up, and rely heavily on the local incident action plans to be followed (if they even have any and apparently they did, Nagen just didn't follow them, i.e.: the school buses being used for evacuation). Any outside resources are to be put into action by the local incident commander.
Having the police looting the local Walmart right alongside the looters, is just another example of how fucked that city was.