Again, it goes back to not having a fare filing (not routing or fare rules) due to the fact that it's silly to fly between PHF and PHL via ATL. Why bother with a fare filing when there would be almost no demand? It's not like this is unique to DL. UA and AA also have routes where there are no fair filings because they make no sense.
I agree, no reason to file a fare on this route, but what has made life difficult for some is the move to restrict end-on-end ticketing on most fares. I. In the old days if you really wanted to fly PHF-PHL you could simply book a PHF-ATL and ATL-PHL on a single ticket. Now you cant do that, or can only do it with crazy high fares. All the legacys, except AS, have moved in this direction.
I wonder if this change has been revenue positive? They certainly reduced the crazy connections for low fare trips, but they are also losing the higher fare PHF-PHL premium passengers.