Don't get me wrong but is a very small airport, serving a small region, and with BFS an hour or so down the road, its maybe reached its potential. Its a bit like Carisle or somewhere.
If you live there and there is one flight a day to London, for the sake of a car journey you can have 4 flights a day, on a particular route.
Maybe if BHD didn't get a runway extension or it was delayed indefinately, Londonderry could be an alternative for FR to launch continental serves(depending on the length of the runway up there).
Your arguement doesn't make any sense. If we follow your rationale, nobody would ever use small local airports. Airports like Waterford, Kerry etc. are a similiar size to Derry but continue to be popular despite there being much more frequencies and destinations from Cork Airport which is only about 2 hours from either. Galway Airport also is popular despite being only about an hour from either Shannon or Knock. Derry is a much smaller airport than BFS, and is much quicker and easier to pass through. This is a major plus factor for many people. Also, tourists probably don't mind the fact that theres only one flight a day, or that it only operates on certain days. Derry is much closer to Donegal and the popular West Coast of Ireland, and Derry city is a tourist destination in itself. A lot of people prefer using their local airport, and cater their schedules around the flight times available.
FYI, FR already operate continental services from Derry, they operate to Alicante during the summer months.