They frequently move ships to where they can maximize revenue. The old Rhapsody of the Sea that was doing 7 night sailings from Galveston is going to Asia now. The Splendor of the Seas has been moved to Venice to do the hottest maket in the industry now, the Med/Greek Isles. They will be moving the Voyager of the Seas to Galveston for 7 day sailing the end of this year. With a ship that size in what is considered a minor market, Galveston, I guess they didn't see the need for another ship from that port. The shorter sailings from Galveston were limited in the ports that they could offer as well. They clould only reach 1 or 2 western caribbean spots, usually on the Yucatan. At some point the same two ports for the same clients had to have lost its appeal.
Just speculating, I don;t have any first hand knowledge of what went into their decision.