Read this from Philip Greenspun and you'll understand:

We did three sightseeing/photo flights over New Orleans.  The first was with Vincent, who oriented me to the area.  The second was with Ernie, who pointed out some additional sights and breaches in levees.  For the third flight, we removed the left door of the R22 and left it with the FBO.  Tony flew from the right seat while I took photos out the open left side of the helicopter.  Flying above the city, you realize what a tough challenge rebuilding is going to be.  Some of the high ground neighborhoods are more or less back to normal, with the exception of blue tarps covering damaged roofs.  The low-ground neighborhoods, however, whether formerly rich or poor, are deserted.  It looks as though a 1970s-style neutron bomb was detonated leaving the buildings and cars, but killing all the people.  No homeowner in one of those neighborhoods is going to be able to rebuild without taking on a tremendous risk.  What if the other people in his neighborhood decide not to rebuild?  He will have spent $200,000+ on a new house in a dangerous abandoned area.

There are two levels of risk involved here. One is the risk the putative homeowner is willing to put up with, based on the level of abandonment. The other is even harder to get around - the level of risk a mortgage lender is willing to deal with in order to fund rebuilding in what's now recognized as flood plain. Sure, it's always been flood plain - but potential lenders have had that fact put in front of them now. Between those two things, I expect that large parts of New Orleans will simply never get rebuilt, regardless of what kinds of aid packages go into the project.