A few possible reasons:
1) Shorter flying time means less fuel consumption which saves them money
2) ...therefore a quicker journey time into London than from Stansted (quick terminal transit - perhaps marketing 'waffle' but also fairly accurate)
3) Perhaps a good deal they may have been given by the airport (?)
At the end of the day they are here, and bringing with them a route that has the potential to grow in the coming years (which Adria have stated themselves). Hats off to the SEN management team for gaining a Star Alliance member.