Stansted serves London, which nobody needs to be told that it's huge for inbound tourism as well as outbound. With that in mind, whatever alternatives there are at Birmingham, East Midlands or Leeds, they're going to be of virtually no interest to people wh currently use Stansted as their gateway to London or indeed their local airport to fly outbound from!
Load factors are about 80% on average for all Stansted flights and EasyJet alone will be very close if not exceeding that level by being one of the best performers in terms of LF at Stansted. People will not pay any more to fly from cities like Manchester, Birmingham or Edinburgh than they would from London-Stansted.
There is no way that there is a lack of people wanting to fly the Stansted routes. If that were the case then EasyJet would have been cutting back years ago. It's also evident that competition isn't the reason either. They have always had Ryanair with a much stronger presence at Stansted alongside them and they put up with BA and the charter/leisure airlines like Thomson and Monarch (who between them offer quite similar routes) from their largest base at Gatwick.
I know damn well why they are doing it! It's because they are doing everything they can to get the very best out of their new Southend base. That is why these cuts suddenly started when they announced this new base and is why the key destinations like Alicante, Barcelona and Faro are getting the axe from Stansted, because EZY now fly them from Southend. EasyJet could regularly fly these routes from both airports and planes would be highly filled and both would bring good profit, it's just easyJet are trying to make their new Southend base look a stunner to attract public and media attention and the way to do that is to downgrade it's biggest rival airport and reduce it from competition. That's why Fuerteventura and Tallinn have now moved to Gatwick, because Gatwick is less of a threat to Southend's performance.