The reason EOS and MAXJET do so well out of Stansted is that they have little or no competition.
Theyre offering routes to people like me who live north of London and would have to drive another hour (at least) and pay considerably more for both the air ticket and parking of I went to Gatwick or Heathrow
Despite political rivalry, if either airline decided to operate from Heathrow or Gatwick, they'd encounter formidable financial costs which they don't presently have.
Slots are one thing, but a considerable increase in operating costs and exposure to the open market are another.