The problem in Belfast is that virtually every airline would choose BHD, primarily for it's proximety to Belfast city centre but it cannot cater for such expansion. As an airport with lower operating costs that is more ideally located, why would anybody really want to choose or prefer BFS over BHD?
I guarantee that flag-carriers from overseas like Lufthansa, KLM, Air France etc. who serve comparable cities like Glasgow and Edinburgh would like to serve Belfast but want BHD as their airport but because it would struggle to cater for such carriers, they just ignore this market and look elsewhere which is more profitable.
Airlines don't think "oh we can't use BHD so we'll go to BFS instead" they are more likely to think "we can't use BHD and as BFS is not right for our business model, where else can we look at?".
It's an even more complex issue becuase you can't really close one of them. I know it's been suggested but BFS can't close because it is the only one that can handle anything bigger than an A320 but closing BHD would probably anger more people. I'd say adopt a similar model that Glasgow has. Extend the runway, terminal and apron at BHD and make that the main airport (like GLA) and then make BFS the secondary one (like PIK)but I know many will be against me on that.