Seems pretty simple to me.

MEM-LGA is more O&D, while any international flights from MEM can flow thru ATL. Why should DL fly from MEM to JFK to connect internationally when they have ATL? Since DL wants to have 2 hubs in NYC (once they work out the LGA deal with US Airways), it makes no sense to fly internationally thru JFK if the flight originates in the South.

Most East/Midwest DL international flow is like this:
South (and general)- ATL
Northeast - JFK
Midwest - DTW