I would imagine that it is because the Fantasy left Port Canaveral about 4-5 hours late after getting into PC very late after riding out the hurricane last night. They probably will get to Grand Cayman late on Monday and leave late on Monday, so will probably skip Costa Maya in order to get to Cozumel on time. They usually have to make some adjustments to their itinerary in situations of bad weather.
Costa Maya is a island in which they only stay a few hours anyway and Cozumel is a much longer stay, so they probably figure skip the island with the fewest hours in order to give the guests a better schedule.