While a mandatory evacuation is a great idea, most of the people that remained behind simply could not afford to leave. That being said, after going through several hurricanes last year, people never think "This is the one, I've got to leave."
Additionally, unless Martial Law is declared, the authorities CANNOT MAKE anyone leave. Unfortunately, it is these folks we are now risking lives to save.
Sometimes it is a no win situation.
Fox News just showed a report from a rescuer pleading with people to evacuate their second floor abodes in N.O. and they still refuse to leave.