Back in the day I worked for one particularly "Skytrain" operator where, in the days when we merely operated DC10-10's, the LGW/LAX/LGW service would fuel stop in BGR.
Once we got DC10-30's the flights would operate direct LGW/LAX/LGW however, what with all the tankering of fuel, it remained more cost effective to fuel stop in BGR.
Longer flights can prove more expensive to the operator whilst less desirable for the passenger!