"could someone tell me why Adria picked Southend rather than Stansted"
a few real reasons:
1) No slots at Heathrow and way to expensive
2) No slots at Gatwick
3) London City unsuitable
3) Predatory carriers at both Stansted (Ryanair) and Luton (Wizz) who would move in straight away if the route started to turn a profit (remember what happened on LJU-LTN)
Adria are a traditional airline desperately trying to survive by finding niches away from the big low cost boys - so this is a perfect fit, Southend will do fine as the London gateway in this case. Southend Marketing department (who are all jolly decent chaps/chapesses by the way) probably couldn't believe their luck (again).
Not anti-Southend, far from it, just re-introducing a bit of reality.