Actually, US is increasing transcon flying for the summer - on May 7, both Philadelphia and Charlotte to San Francisco will go from three to four A321 flights daily. Oddly, this means that PHL-SFO will have more lift than PHL-LAX!
It is interesting that SFO has more nonstops than LAX. Added to that is the fact that unlike CLT, PHL-SFO also has 3 UA nonstops (total 7 nonstops) and 2 UA PHL-LAX nonstops (total 4 nonstops). The LAX-SFO difference might be mostly due to the US-UA codeshares to Asia via SFO.