Caribbean people are not travelling enough
The bottom line is simple; people in the Caribbean are not travelling enough through out the region and intra-regional cargo is limited. The Caribbean is very far from the recession recovery. Despite the new routes and the fares might be low, it is still costly to take a vacation in the region (accommodation, food, spending money etc.) and people are watching their pockets in these times or prefer to travel up north (US, CAN and UK). I personally have travelled on the RD, BW and LI recently and the loads were very light at that time in the winter season. No matter the airline or aircraft if you don't have enough payload it becomes a scenic flight.
NightWolf