Not sure. BA3 is pretty useless now to a lot of people due to the lack of pre-clearance at SNN so it's probably the better option to head to LHR.
Might be Brexit related as well.
BA4 won't operate if BA3 doesn't fly to JFK in the first place.
Always possibly the spare A318 will be used on a Middle Eastern route or there may be a demand for somewhere else on the east coast?