Fixed-Wing pilots/captains traditionally sit in the left hand seat and therefore the left hand circuit is the norm as it gives the pilot a better view of the runway. My guess is (and it is a guess) that by having the island of a carrier on the starboard side then the pilot will get a better view of the deck on a left hand circuit. While most carrier aircraft are single/tandem seat, and therefore it doesn't matter, the pilots will have been trained in the tradition of left hand circuits.