I'm not strictly sure of the situation in NO, but hurricane science usually is not able to give enough advance notice of landfall locations for large scale evacuations. In other words, you usually would have to make the decision to evacuate before you really know whether or not it's necessary. A heavy decision with a large metropolis.
I suppose the real question is why there isn't a plan in place to plug the levee. It's not like this scenerio hasn't been tossed around for decades.