For Stansted, many popular routes are now only left with the low-cost but very basic service of Ryanair and Monarch could benefit by bringing something new to Stansted since Essex, Cambridgeshire and Hertforshire are all very affluent areas and would all desire something with a little more service from their local airport
Stansted is not a popular airport with many travellers. It has good facilities and is uncrowded, but it just isn't where people want to fly from. Hence it's major carrier is Ryanair, whose main attraction is price. If you want to fly cheap, you go to STN, otherwise you go to a more convenient departure airport.
If MON went into STN, they would be going head-to-head against the lowest-cost and most aggressive competitor, for a share of a tiny market (that area they don't already serve from LTN or EMA).
So, let's face it, they won't be going in to STN any time soon.