Because years ago, for reasons I can't fathom, BA decided to withdraw from the regions and concentrate on LHR. They were forced to keep Gatwick because of capacity limits at LHR but have remained true to that decision. I realise that LCY based Embraers have operated weekend fillers from MAN but thats a long way from a longhaul base like this.
They became "London Airways" and thus Aer Lingus have spotted a potential business which IAG have allowed..........