I'm not in the airline business so I'm just spitballing, but I would suspect you capture more business along routes like London-LA if you have a variety of departure times throughout the day. While the total volume might make sense to fill an A380, you'd lose business to somebody with smaller planes but more scheduling options. Again, just spitballing, but I know I probably wouldn't delay or move up my departure for a few hours just to get on an A380 if there's an A320 that can get me there and leaves at a more convenient time.