Roatan has only 2 US carriers landing there. Continental on Fri Sat Sun and Delta on Sat yet the price can be lower than to BZE. AA and USAir do not go to RTB.
A few years ago in Grenada the hotel/condo rentals etc had to guarantee so many sold seats for the US carriers to start to fly there direct. Now they have a tourist industry, not huge but growing.
Maybe the GOB does not want mass tourism to take over it's cayes and that's why they don't figure out a way to lower prices.