Because raising deposit rates by 25bps isn't going to lead to a flood of new money pouring in. Especially when you consider banks with reputations of being 'safer' have done the same and the rates offered by AIB are nothing special. The only thing I can think of is that it was a defensive move to stop deposits flowing out of the bank. If that is the case, there would have to be serious concerns raised about the 'stickiness' of the deposit base in AIB. And to be honest most deposits in there at this stage would be retail based and would not be as rate conscious as corporate so if people are still saving there, they would be likely to stay there even if AIB only passed on 10bps of the increase.
I am just surprised they didn't try to take this opportunity to improve their margins. Also, if I was the Government, I would be asking why don't they not give the full 25bps to depositors and then they might have room not to pass on the full 25bps to variable rate mortgage holders. Again, I don't know anything of the AIB deposit book. Maybe the majority of their deposits are fixed term so this rate rise only affects a small part of their deposit book.