I have to agree with what TriStar500 said. Quite a few people from Austria got used to the low-fair concept that, for example, Sky Europe offered from Bratislava and they were willing to travel the 60 km. in a bus to get to the city and fly cheaply.
Obviously, that couldn't be of Vienna's airport liking, so they simply bought it. Already in October BTS
catered for more than a million passengers, which is quite a lot for such a small country (and capital city).
I don't know how much truth there is in this, but when I was last in Slovakia (about a year ago), rumors had it that Vienna airport wanted BTS
mainly for cargo and that they were planning to terminate all passenger operations there. However, I imagine they couldn't do that now, since the place has been growing massively and the low-cost market is a pretty lucrative one. Not mentioning the protest that would surely follow after such a step...