I don't know why is should take years. For at least 300 years, since the founding of the city, every civil engineer who has looked at NO has predicted this very event. It's pretty obvious. You've got a city built in a mashy hole surrounded on all four sides by water 9 feet higher than the city water table. Going back at least 50 years there have been several studies concluding that NO was the most likely spot in country for a cataclysmic natural disaster. What went wrong was that for decades everyone concerned has ignored the problem until the big one finally hit.
Oh, not why the city flooded. Why the disaster planning and initial response were so piss-poor.