One thing I would point out from a demand point of view is that SNN has transatlantic flights operated by all thee major alliances. BFS currently has no transatlantic hub connections. A substantial number of BFS-EWR passengers used the service to connect to other US/Canadian destinations, meaning Air Canada would benefit from these connecting passengers more so than they would if operating to SNN.
And to rephrase what owenc was trying to say; BFS has a 60 min catchment area of 1.4 million and a 120 min catchment area of 2 million people. SNN on the other hand has a 60 min catchment area of 0.58 million, and a 120 min catchment area of 0.95 million.
And let's not forget as well that when BFS was last served from Canada, the majority of passengers were Northern Irish people travelling to Canada. Belfast (and NI as a whole) is now becoming more of a tourist destination, meaning passenger numbers would be less lopsided.