Lots of reasons SK may not be matching BA:

1. SK has had strong sales already and flights are well booked at higher fares, thus more profitable
2. SK has far fewer seats each day to fill across the Atlantic compared to BA, thus BA must discount more heavily
3. SK service is non-stop and there is always a premium applied to flying non-stop versus having to connect at a non-national carrier's hub
4. Have you ever flown BA's back cabin compared with SK's? You might want to pay a bit more on SK if you have been on a full flight on the British carrier.