because there is only one direct continental (or International) scheduled route from Belfast (i.e. AMS with eJ), a lot of people use the low fares airlines to get to London, and get onward connections to Europe/World with other carriers.
This creates massive demand for the London Routes.
Belfast people are sick of travelling to Dublin for direct (over priced) flights- as well as outrageous parking charges.
As I mentioned in a previous post, there is a great site for making low fares connections, sorry to harp on, but, I find it brilliant-
openjet 