Pretty sure its a financial thing. Carnival brought in the Conquest after Katerina and the smaller ships of the other lines sailing from Galveston have been selling much cheaper then before. RCCL is pulling their smaller ships out this winter and putting the Voyager instead. Maybe Princess doesn't have a spare large ship to put port? NCL is in its last season with the Dream out of Port Houston...
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, Celebrity Silhouette 2013