My opinion is that the problem was mainly that FEMA kept following the plan they had on file even when it was apparently not working. I think both key failures, the bad plan and bad leadership in response, come from the scale of the problem. The disaster caused by Katrina was different in kind and was treated as only being different in degree.
So soon after the embarrassing failure of emergency response it’s a little risky to venture an explanation. The appearance is that government planning for emergency response to “the big one” failed to include providing food and shelter for those stranded by it.