In case you didn't remember, it took 4 hours to get across New Orleans the day before the storm hit. Not out of the danger area, just out of the city.
Probably a couple of things happened.
A lot of the drivers were probably heading out of town.
They didn't think the city would flood
They also didn't think to or chose not to load them full of evacuees and drive out of town, or didn't want to add to the traffic load.
People didn't think it was really going to hit until it was really upon them. Too late to do some of these things.
Again, this was because they did not use the inbound lanes of the interstates for outbound traffic. They do this in Florida every time a hurricane threatens.
Incompetence is the reason. Nothing else.