Birmingham is only stronger the Manchester now because almost half the Birmingham flight are taking the train to London thus using it as an interim London service.
Or, the WAT market is stronger to the UK Midlands than the London market, so it makes sense to offer the service to Birmingham, and for London passengers (shock horror) to travel NORTH to catch their flight.
In the midlands we've been doing that in reverse for years, and will continue to do so far the foreseeable future!