That's the problem with Galveston, between Galveston and Houston terminals, there is plenty of terminal and dock facilities for more ships, but with the distance involved, they can really only effectively do western Caribbean cruises so maybe they feel its not worth putting more ships there... I am sure they could fill them or else they couldn't be charging the premiums that they do now.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007