Because he royaly screwed up. But his mistakes, however awful they were, don't excuse the mistakes made by FEMA.
It wasn't Nagin's fault that a search and rescue team from Virginia had to go home because they couldn't be cleared into LA. It wasn't Nagin's fault that resources haven't been released because FEMA can't get the paperwork done. It wasn't Nagin's fault that emergency vehicles from Florida couldn't be brought into the impacted areas because FEMA and the GSA couldn't agree on how to handle it.
I'll keep saying it, government across the board, city, state, federal, was completely unprepared for this sort of event. That's inexcusible. At all levels.
Read the tag line.