@EI-BUD propensity to use choice A over B is measurable at a segment level. If someone is using a Belfast Airport flying in on business their more likely to use BHD than BFS for reasons of convenience and speed. This added to those going the other way which may also have a BHD bias means BHD likely has a higher yielding traffic profile than BFS. I note all the good points about moving back to BFS involve saving some money.
As to BA leaving this to EI or vice versa, well the Oneworld London routes still see AY on HEL and IB on MAD, so no sign of that happening yet.
BMI moved to BHD because their passengers wanted it, the airline serves the market not the other way round. Remember they even served BFS again when BA left and dropped it soon after.