My view, fwiw:
Most pax to/from LHR actually want to go to London or the surrounding area. They are also more likely to sit at the front, in the seats which actually make the money for the airlines.
MAN has a pretty good long haul network for a regional airport, but the demand is more for economy than business/first class, and the airlines don't make much from that.
In a nutshell, any longhaul destination to the UK will yield more £ to/from LHR than MAN because more pax come from, or want to be, in London in the first place, and they're more likely to buy a premium priced seat.
Exceptions are to/from Pakistan, due to the large immigrant communities in MAN's catchment area, and Emirates et al which seem to do quite well ex-MAN.
Over to the airline boys n girls for a better answer than this one.
