LAND AFTER: The first flight out of LON to LAX leaves at 0930. The last (Air New Zealand) at 1625. That's not too far off 1800. In any case GalleyWench is right. It all comes down to maximum aircraft utilization and connecting pax. Why have two aircraft spending 9 to 12 hours on the ground each when you can use just the one inside a 24 hour period.
Incidentally, the first out of LON to NYC is at 0800 and the last at 2030! And finally, if you take the 1800 UA to JFK you can connect with a DL flight to LAX, arriving at 0105!!