easyJet fly to only 25 destinations from STN, of which 20 are (or will be) in direct competition with at least one airline. That means they'll have to reduce prices, reducing profits, in order to compete. It does not make sense to wage war on Ryanair's turf, all for the sake of being the second largest airline at STN, because easyJet will lose that fight. If Jet2 want to expand their STN operations, easyJet will watch from the sidelines because they know that Ryanair will crush the competition at STN. What does make sense for easyJet, however, is expanding at airports where easyJet is already dominant, and where the airline can make the most money. The same goes for Ryanair at LTN and LGW - they aren't going to expand operations at these airports because they know that they'll lose the fight against easyJet. The fact the Ryanair are moving BFS-LGW to BFS-STN is evidence of this. STN is Ryanair territory, while LTN, LGW and SEN are easyJet territories.