I imagine, too many seats (to LON in general), too low fares and too high costs into LGW. Even if they fill 80% of seats it doesn't mean that their yield is sufficient enough to make money.
I've been using them on the CGN-BHX service quite regularly. Loads average 60%. Unfortunately for me, they're changing the times with the Winter schedule and I'll have to go back to using BA from DUS.