Interesting that there are mid evening departures out of BUR and LGB but not LAX. I checked flightaware, and there are only about 6 or 7 departures out of PHX from 9:40 pm onwards - DEN, ELP, TUS, EWR, PHL, and CLT.
My guess is that folks flying out of BUR and LGB would be flying to PHX to connect to the east coast whereas there are already non-stop flights out of LAX to all of these cities (save for probably ELP).