The mayor was probably told to not evacuate so the mediadnc could blame Trump.
Maybe,
But where to 6 million people go? What do you tell the employers of those people? What happens when the storm DOESN’T hit after you evac? Will those companies be forced to keep the people that evacuated? Will the city reimburse them for the lost time and production? What happens when I evacuate, following the order, and get laid off because I didn’t stick it out?
I read something post Katrina that no major city will ever order an evacuation. Just not politically (or at times, physically) possible.