One big factor is that the flights have to depart the US west coast late enough for pax to make connections from other US cities. Departures before 11am or so would eliminate many possible connections. The same logic applies to departures from pretty much any hub (look at JFK, IAD, etc.; most flights to NRT & ICN leave around noon).

Later departures would make connecting in NRT, etc. difficult.

If earlier landings were permitted at NRT (I believe 6am is the earliest allowed landing), then airlines could do midnight departures from the US. A few flights to ICN leave around midnight and arrive around 5am.