I think that part of the problem, and will probably get ripped apart for this, is the lack of inspiration from the travel trade and travel agents. If you pop into a travel agent and ask for a beach based holiday direct from BFS, they will tend to recommend traditional Spanish costa's or Canaries etc. To holiday in Greece or Tunisia, you have to tell the agent thats where you wanted to holiday, rather than it being suggested.
And to many of the general public they may not be accurately aware of the choice of destinations direct from Belfast, solely relying on the agents advice and recommendation. With the small population size of Northern Ireland, destinations will always be somewhat limited, but a bit of variation and diversification wouldn't go a miss.