PHL has a ton of US loyalists that probably never gave VX a try. However VX suffered from a stiff competition at the beginning (including a drastic increase in capacity) and never got a foot hold into the market. Before VX entered PHL-SFO, US was at ~5x day and UA was at 2 to 3x day. VX entered with 2 flights, US added 1 and UA added 1, and for one summer there were 12 PHL-SFO flights. Now this upcoming summer it is back down 7. Maybe if PHL continues it's growth as a tech, healthcare, innovation hub there will be increased demand between PHL-SFO.