A quick random search shows that DL already services PHL from their other four hubs and some of those are on Delta Connections. So I'm no expert on routes but I'm sure Delta evaluates their sales on the route and while a nonstop might sell better than the current offerings, it's probably that using a takeoff slot, gate, etc. aren't worth it compared to sending a plane to ATL where the few Philly pax can go one way and all their KL/AF/DL international departures can fill widebodies.