I'm certain it's a safety issue as much as anything. The water in the Gulf is rough, and the weather has called for higher seas and winds, which would make transferring people somewhat risky. They would also have to bring an empty ship alongside, which they probably don't have in the area right now. By the time all that happened, they'd be in port.
In spite of the alleged horror show on the ship Carnival has stated that each passenger will get full compensation for this trip, transportation costs to their home port and a free cruise in the future. Maybe it's a small price to pay, but there it is.