Historically, PHL-California demand is big in the summer and much less so in the off season, particularly the first few months of the year. I was actually surprised that late flight was running outside the midyear peak months. SFO in past years has also seen reductions in the off season.
I suppose LAX as an AA hub changes the dynamic slightly, but with 717 scheduled into LAX just before 1130pm PT (at least for today's flight), there can't be much if any connecting traffic.