It has nothing to do with the airport operator. Ryanair have maintained a good relationship with the airport. The problem lies with the public inquiry into the runway extension at BHD. It is a prolongued process which in effect has no timescale. The underlying reason to opening the Belfast base was to eventually allow flights to/from Europe. They believed this was possible as the runway extension had been agreed in principle, then it was stalled due to an inquiry being requested. So in true Ryanair fashion, enough was enough. I'm sure there was a bit of "we aren't getting what we want so we're not playing anymore" however if Ryanair can't put a timescale to their expansion plans at Belfast then more economically viable ventures may indeed be elsewhere?
They stated they would readress the base if circumstances allowing expansion were to change. Additionally they could have operated flights to BHD from other bases within the UK. Who knows why the won't?