In the 90s and noughties, Stansted took a great deal of Luton's traffic (TNT, Ryanair, etc). This was achieved through cross-subsidy by BAA from the more lucrative Heathrow and Gatwick, and happened despite Luton being better positioned for the north of London catchment area.
Now that Gatwick has been freed from the dead hand of the BAA, and itself has capacity as a result of the recession and migration of transatlantic services to Heathrow, it is winning business from Stansted. Stansted can no longer rely on what are now illegal cross-subsidies, although it may benefit from sharing ownership with Heathrow.
The most attractive airport for London is Heathrow, in terms of catchment area, proximity to the centre, and connections. However, it is overcrowded, expensive and slot restricted. The next most attractive in terms of catchment area is Luton, but it is far too small, and is not being managed well due to its legacy owners (Luton Council). Gatwick has a slightly smaller catchment than Luton, but is in a wealthier area and has more capacity. Stansted has the smallest catchment area, and land communications are mediocre. However, it has both capacity and the ability to construct more.
It's no surprize that the airport of choice for London is Gatwick, and Stansted probably has most to lose under the current ownership pattern. The only way Stansted can grow will be after Gatwick is 'full', and by competing on price to fill its underutilised facilities.
Otherwise, look forward to further reductions in traffic.