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.
If nothing else, because ORF and RIC have routings permitted on Delta.com. I don't think PHF-ATL-PHL is fundamentally any "sillier" than ORF/RIC-DTW-PHL (it's something like two hundred miles longer than ORF-DTW-PHL but that's what? 20 minutes of layover worth of difference...or, more bluntly, morning traffic on the HRBT). It seems that there was, frankly, a blinkered attempt to wipe out Virginia-Atlanta-Northeast routings as much as was possible.