Yes, they considered Luton before - when they first came in to the UK the flights appeared on the Luton Airport programme for a few weeks, so the decision must have been close.
And Buster's right to be cautious - not just Volare, remember Helvetica?
But there are some reasons behind this:
Stansted has increased its charges, and they will go higher.
Sky Europe, although a public company, is one of the weaker locos. Its fleet has stagnated at around 14 for some time whilst it achieved cost and operational efficiencies (see investor section of the website).
On Bratislava - the key route to its main base it competes head-to-head with Ryanair at STN, with similar flight timings. On Prague from STN it would compete with EZY and CSA daily, whereas from Luton it's just TOM three times a week.
Operationally, I'm sure Luton is more efficient to achieve rapid turn-rounds.
And maybe, just maybe someone has realised that Luton is far better located in the UK than Stansted!
It will be interesting to see how Poprad and Kosice perform - neither has any current direct link to Western Europe, and the former's website is mainly about private flying!