This may help shed some light on what a cruiseline does.
When a cruiseline starts a new itinerary, it usually an smaller vessel that is sent. In DCLs case, its the Magic. Theres a few reasons why, but primarily its the size of the ship.
When a cruiseline tries out a port, theres a lot of uncertainty. (I had this discussion with my son last night). They arent going to send a 4000 room ship somewhere if they arent sure of the dembhand. So they send a smaller ship to test the waters. If it works out, great. If not, as in the case of Galveston, DCL wasnt trying to fill a 4000 passenger ship, they had a 2500 passenger ship, so there was some safety factor built in so the ship would make money, as opposed to sending say the Dream there, and losing money because of the lack of passengers onboard. (Its easier to fill the smaller ships plain and simple)
2nd, is that the smaller ship may have outgrown the current port. Say PC for instance. If DCL was selling out say 95% of its cruises year round,(Im just making up the number) on the Magic class ships, it would make sense to put the bigger ships there to
A. Satisfy guest demand,
B. Give DCL the option to try something new. In this case, DCL tried Galveston. Unfortunately it didnt work (At least it looks like it didnt).
I think what we are seeing is DCLs attempt to find new homes for the Magic class.
DCL most likely has a 5 year 10 year and 20 year plan. When Mexico started having issues, and the Middle East threw a monkey wrench in to the Med plans, DCL was forced to find places for the Magic class to go, and accelerate the plans. Alot of this is probably earlier then DCL would have liked.
3 ships at Port Canaveral for example. The Dream and Magic are basically sharing the same itin. It worked for Carnival. A ship in Miami for half the year. (Granted the itins are the best, but DCL can guage the likelyhood of guests traveling to Miami. )
I think in the 2015 itinerary well probably see more wide spread carribean itins.