There are pharma/biotech company ties between SFO and PHL. It also exists with RDU.

I'm not sure what the economic industry ties exist between PHL and LAX, but there are likely a lot of companies sending business pax back and forth.

I wouldn't be surprised if VFR is greater since Southern California has more people living there than the Bay Area has in it, by metropolitan size. The Bay Area is more destinational as a leisure attraction but that's subjective.

I think UA is content with pax connecting in one of it's hubs to reach LAX from PHL, and those that want UA nonstops can just head up to EWR.