Nothing to stop them at all but remember too that for many years all flights to Belfast were to Aldergrove and loadings have been consistently good. If folk want to go to Belfast and City is no longer an option, they will just go to BFS. It is still easier than travelling to another airport in Lancashire just to save a few miles when they land.
Easyjet totally changed things on LPL-BFS route. Before they arrived it was operated by Manx Airlines to Belfast City using APTs as a rule, then I think BA took over Manx they used J41s from Memory. High fares ruled with limited choices of departures.
Now it is Belfast's busiest route outside London ( around 500,000 per annum) with multiple A320 departures ( in normal times) and much better priced fares.
Change of airport in Belfast has not been an issue on passenger take up.