I think the reason was a combination of ending the afternoon preclearence at Shannon (apparently due to lobbying in Washington by a competitive major US carrier, who presented it as US taxpayer expenditure giving an advantage only to a foreign carrier) and the end of the major bulk purchase contract that started the service off, from a major Canary Wharf/Wall Street bank.
I was actually surprised that the remaining service was the early one, leaving at 0940. This means that the much-vaunted "work at Canary Wharf before the flight" is not possible. UK passengers will be coming directly from home, for many senior execs who live out in leafy Surrey etc possibly more of a nuisance journey than getting to Heathrow. US passengers will come straight from their hotel - they might as well have gone the previous evening on the last Heathrow departure.