You can't just cancel a service if one way you have a small load - say 35. What if you are booked to be full on the return leg - what do you do with all the punters there? Long haul flying, and particularly the states, is quite fickle and seasonal at times, and it is not uncommon to have really bad loads in one direction for weeks at a time, while coming back the other way the aircraft is 100% full.
Also, V have done well out of the QF380 dramas with lots of pax being sent their way I believe since other QF services are full. Then again, QF also did well picking up V pax when delivery of their aircraft was delayed.
Such is aviation.