There are ATL-LGW, EWR-LGW, DTW-LGW and PHL-LGW because are hubs of DL, CO, NW and US.
Their connecting passengers don´t have the chance to choose between LHR or LGW, except for those travelling through EWR because CO codeshare VS EWR-LHR flight, but still not much of a chance.
Try getting a ticket MSP-DTW-LHR, the MSP-DTW leg on NW and the DTW-LHR leg with BA and it will be very expensive, its cheaper is its all NW flights.
The question here is why VS flies to LGW from MIA instead of LHR ?, maybe no slots available ???
I think that DL people are smart and they know a JFK-LGW will not succeed, specially with competitors as AA, BA, VS, DL, AI, and KU. The last two are not as important as AA that has quite an impressive hub at JFK.