A lot of folks were ready, but what would be the point?
Air tankers lay retardant to corral or direct a fire, tie together anchor points and natural barriers to prevent fire from spreading or to allow it to burn itself out. As the city was flooded, where was the fire going to spread? Individual structures burning surrounded by...deep water. The fire department couldn't hope to do better. Most of those places are going to have to be gutted and torn down due to water damage anyway...is there much difference between it burning up, or developing mold and getting torn down?
Retardant contains arsenic, among other things, and is highly toxic to aquatic life. We have strict regulations about how close to water we can drop. Fixed wing air tankers might not be the best soloution to fires in the New Orleans area. Many of those fires are structure fires, with interior attack required. Putting retardant on the outside of the house is a little like rubbing suntan lotion on a cancer patient. The fire is already inside.
Setting up tanker bases takes time and requires equipment. Space and expense used to operate fixed wing air tankers would be counterproductive, especially in light of the need to fly supplies in and out, move people, equipment, etc. Helicopters are already present, and there are much bigger priorities than fighting individual structure fires in isolated structures surrounded by water.
Otherwise, I'd have been happy to take a tanker down there. We were standing by and available.