Pilot basing might be involved -- see this thread Delta cuts seniority requirement on 757/767

Sounds like they don't have enough 757/767 qualified pilots at JFK base (not sure if there are any based in SEA/LAX/SLC, etc), but there are enough qualified pilots to cover flights on this equipment that are based at ATL and DTW. These planes are not going to be around for that much longer, so there may also be some pilot base consolidation going on. Note that while D1 fare filings have been completely pulled on JFK-SAN/LAS routes (on non-stop flights), they still exist on JFK-SEA (for flights flown non-stop with D1 equipment -- see https://www.flyertalk.com/forum/34955457-post4.html). So there still might be some hope DL will restore D1 flights to JFK-SEA at a future time.