Carnival does a lot of the 3 to 6 night trips out of Florida, South Carolina, Baltimore MD, New Orleans LA, Texas, and Los Angeles CA. This makes them the leader in the short cruise segment at low prices out of more US ports then any other line.
As Johngold mentioned, I live in Los Angeles, I take the short 4 night cruise on Carnival often, just as a cheap get away. If we do not go into the ports we still have fun and relax on the ship. It is less expensive then a road trip. This gives Carnival a demographic different then any other line. It is not Celebrity by any means, but 4 nights from a local port for about $ 200.00 each, how can you not like that ?