BRS not only has the Bristol, Somerset and Gloucestershire market to itself but the whole of South Wales and all of Devon and Cornwall too (given what little the likes of Cardiff and Exeter offer). That is a huge area.
LBA not only has MAN to contend with, but the area BRS's catchment encompasses would include LPL, NCL, MME, EMA, BHX and HUY if applied to LBA.
Yes, the North has a much denser population but lots more airports, one of which acts like a giant greedy black hole. No doubt LBA should be handling far, far more, but it's way behind thanks to generations of luddite politicians and it'll take years to even start catching up the competition in any meaningful way, and like it or not, MAN will make it very hard to attract many of the airlines and routes that we would all like to see at LBA.