People from North London, the South east and beyond would rather fly from STN rather than LHR or LGW remember LGW is at least 1hr 15min from STN and LHR is a little more, so why not not give passengers a choice .
Except American, Maxjet and Eos all gave up or went bust as the yields weren't there. I assume you also have a cunning plan to rebuild Stansted to take 7 based B747-400s on turnaround, about 5 at the same time.....
As has been mentioned before the flights are where they are because the market ( and the passengers ) want them there. No one is stopping anyone offering flights from Stansted. The simple fact is as a business, your yields are higher at LHR and LGW. Airlines do not think everyone lives South of London, you are distorting the reality.