I suspect it's more about slots and aircraft utilization. There aren't many domestic DL arrivals into JFK that late in the evening and it's of course too early for the redeyes to arrive. For me personally I'd certainly consider it as an option with a well-padded connection time, but I am not most people.

The inbound aircraft appears to be the VS25 20:05 departure from LHR and arrives at JFK at 22:50. That arrives too late to make any onward connections from JFK. But it allows NYC-based travelers to get a full day in London and sleep in their own bed at night. I suppose the return would allow UK-based travelers to have a full day in NYC, but probably still sitting around JFK for a few hours which unless I had lounge access I would not fancy.