Two things affected the Belfast City, firstly, it's one of the few routes with close competition from Easyjet and secondly the flight times were awful, useless for business and not much better for the 'trip to see the family and friends.' Getting into Belfast at 2115 with a return at 1830 the next day was a bit restrictive (half a day at home and one night in a hotel as against two full days and a hotel if flying with Easy). New flight times should see this improve. There is also the restriction on loads for the route so it will never look great.
As for Knock, it's time they had another sighting, that'd have the pilgrims filling up the planes.