By keeping it in business configuration it would be available for charter to BA to cover when G-EUNA is undergoing maintenance or has tech issues.
I have flown on the morning LCY-JFK service and it was almost full on the day that I used it. I understand that the afternoon service was not attractive, and therefore not profitable, as US customs clearance at SNN is not available in the late afternoon. Having the ability to clear US customs and arrive in the US as a domestic passenger is a major plus point.