To some degree, you are correct. However, factor in the elderly and mentally incapacitated. Factor in the people without mode or means of escape. Factor in stupidity of "leaders" who did stuff like... oh, kept the Ponchatrain bridge traffic flow going in usual direction instead of opening both lanes to get OUT.
How would an evacuation order have affected these people? It's not like the city has the means to bus the people out of town.