Sending the military (active duty or reserve/national guard) after looters represents serious legal issues. That's a law enforcement issue and should be left to the civilian police forces. The military can be used to restore order but it has to be done carefully, as discussed here.

In the immediate hours after the hurricane, it's a natural desire to devote all available resources to search & rescue. After all, people's lives were literally at stake. However, by pulling almost all of the police department away from law enforcement, it opened up the city to those looters. Various military units, especially the Coast Guard, were involved in rescue operations from the beginning. As more personnel and equipment become available, it'll be easier to have the police revert to their primary law enforcement role.

The military is responding as rapidly as possible considering the logistics problems. It doesn't do anyone any good to just dump thousands of military personnel in an area without the necessary logistics support (food, water, supplies, etc.). All you'd be doing is increasing the demand on the emergency workers. A disaster of this scale represents huge logistics challenges. Both of New Orleans' main airports were closed due to flooding. Many roads and railroads were closed due to flooding and debris. Fuel is in short supply, so it has to be brought in along with everything else. None of this is easy.