Scheduling, it just is the way things have worked out, Nearly all West bound flights to the states from Europe leave during the day, while all East bound flights leave at night, thinks have changed a bit, but thats the way it is in most cases. Same goes for Europe/Asia flights, all of them arrive very early in europe and leave late that night. They don't spend to long on the ground in Asia doing it this way. Anyway, airlines would like to change it, Qantas requested the CAA to allow BA flight crews fly the parked B747's in LHR during the day on busy europe schedules, with Qantas taking some of the profit etc..
The CAA wouldn't allow it, but it shows that the airlines want to use the aircraft as much as possible, but with pax convience/scheduling etc.. it doesn't always work out.
Aircraft I've flown: A300/A310/A320/A321/A330/A340/B727/B732/B733/B734/B735/B738/B741/B742/B744/DC10/MD80/IL62/Bae146/AR