Not sure if you mean Virgin (in regards to PHL-SFO/LAX) or Frontier (in regards to PHL-DEN). Delta doesn't compete on either of these nonstop.

Either way, a lot of pax flying PHL-DEN were connecting to the West, when Frontier operated it and as Southwest and United operate it. When Virgin offered 5 r/ts to the West, and USAir lowered the prices, fewer pax needed to connect in DEN, so it was the weakest at the time, Frontier, that bowed out.

Southwest operates one PHL-DEN-SFO (no plane change) and demand will surge again on this route. Regarding Frontier now, my guess is this might help ILG-DEN frequencies and Frontier ensuring it offers connections to the West. It'd be nice if TTN was somehow involved. Maybe it does some TTN-CLE-West and adds PIT, and does TTN-PIT-West, but I'm not sure how it will do it, esp. with Frontier's move away from connections. Transcons from ILG directly like 3x to SFO, 4x to LAX would also be possible but be a bit more riskier.