I fly to LAX pretty often from the east coast. My favorite connection is PHL-LAX but during the winter months, they cut back every year on flights.

Even though every flight I'm on is mostly packed from PHL-LAX and back, I normally see about 2 flights pulled off the schedules each year around late Oct until March.

Usually they have 2 that leave early in the AM from PHL to LAX, but recently has been one. On the way back, there is normally 2 that leave after 12pm, but there has only been one, and I've also seen them use a A320 vs A321.

I asked a pilot one time while we were waiting to board. He told me during the "slower months" they pull those routes and planes and use them for more international travel.