johnnychips, maybe the reason is the locals have discovered that most of the BA European destinations from LGW have direct flights from MAN and the ticket price for a direct flight from MAN in most cases is lower than flying through LGW. Only five of the 29 European airports BA fly direct flights to (together with its codeshare partners) the LCCs at MAN don't cover. In reality there is only one short haul destination that is unique for LGW and it's Alger (ALG), but then LGW don't have Tripoli as MAN has.
Here are the LCCs' destinations from MAN
As said above the real unique flights from LGW is the 12 longhaul destinations BA and its codeshare partners fly to, but it is only 9 of these that don't have direct flights from MAN.