We asked this very question when we took the galley tour on our cruise. The CM (I forget his title - one of the head chefs) said that menu changes are a major undertaking logistically and that changes happen very slowly, like one menu item at a time. The coordination of food suppliers and quantities requires precision since the ship is in port for such a short amount of time every week and there is little room for error. Once the supplies are on board for the cruise length, that's it. No emergency deliveries possible if they run out of something.