If it was a serious emergency such as risk to life Belfast would of been chosen but when it's not the airline will pick where is best.
The BA routing was to the south of DUB/SNN so BFS would never of been considered.
As for airlines not wanting to use a particular airport as Gulf Julliet Papa is correct but cost may also play a role as the chances are a diversion by BA to Belfast would cost a lot more as they are not regular operations but it's likely they will be charged normal fees at DUB applicable to B747.