Would you believe they were not expecting the levee to fail?
Whether or not they should have, they didn't.
If you accept that, all else is clear.
It isn't really clear there is "incompetance" here, and I'm sure many FReepers could have done better - but resources are always limited, and officials all up and down for nearly three hundred years have made certain decisions regarding New Orleans that contribute to this disaster. Katrina herself contributed a mite, too.