RoyHudd people tend to use protected connections, LGW is mainly a p2p airport although DY are changing that. It's not a hub for anyone. You are misunderstanding the meaning of the word.
It's easyJet's largest BASE. BA's second but predicated on leisure point to point, hub dismantled by Rod Eddington. The examples you gave all have a willing hub carrier at one end, no one does that at LGW. This is exactly why no one picked up MAN-LGW, the market is simply not commercially viable at the moment. If Norwegian were to grow massively then perhaps a MAN feed would be an idea but that would dent their ambitions for MAN p2p!
If it was as simple as you assume, who is going to fly it and why aren't they already doing it? LGW has a similar demographic to MAN so puzzled as to why you want to feed another London airport at the expense of MAN going direct?
easyJet historically don't do connections, it adds fat to the cost base. Now they keep talking about changing that but beyond baby steps, they're not going down that road and with Brexit impacting them in a very particular way, adding even more costs to a succesful business model at a tough time may not be ideal.
Finally ask bmi how much money they make in connections? Ask BA short haul how much % revenue they get on a connecting passenger to long haul? Even flybe are not exactly coining it in on that score I believe, it's hard to get right.