It hasn't been explained, but the Dome refugees disproportionately were of the lowest sort - drug addicts, gangsters, alcoholics.
I can stand corrected, but the "dry" areas are on the lip of the Mississippi river, the high ground, the French Quarter, The CBD, and the poorest area of the city. All the family residential areas were wiped out, except maybe the high ground hugging the Mississippi.
These people were not at the Superdome. They were at the Morial Convention Center.
From the stories they have been telling, these people were largely those who weathered the storm at home and were told to walk to the Crescent Connection bridge to find transport.
There weren't any buses, so they found shelter at the Convention center -- which was being used by the dregs of society.