Tourists also make up a significant number of the refugees.  No cars to rents, no extra flights out of the city.

Why the Superdome?  It's a big space with emergency power and which was designed to survive 200 mph winds.

The current problems with the roof indicate that the engineers may have been wrong about that, but it was still the safest shelter they had.

I think most of the tourists are staying in hotels.  Hotels got a waiver from the evacuation order for this reason.  They are putting people on the middle floors: above the storm surge, below the worst of the winds.  They hope, anyway.
Right, but government is supposed to help those in the most need and good people should pitch-in. How many are at the Superdome? 20,000. If they filled every bus, truck, train, etc they could have cleared them out within a day.
I think they did try to get people out.  But they had to have somewhere to send them.  And the traffic alone was a big problem.  They actually did a lot better than I thought they would, given how late they waited to sound the alarm.  

The problem with moving the frail and elderly is that the journey can be more stressful than staying.  Three nursing home residents died on the bus while they were being evacuated.