Could also be something to do with capacity at KRK. KTW is a much bigger airport than KRK and may have spare capacity where KRK haven't?
And KTW is a big Wizz base - maybe FR are trying to wind them up?
Wizz Air have stopped expansion at KTW because of high airport charges. (I herd that a while ago) It was only a few months ago Ryanair were pulling all routes from Katowice (except Dublin) because of airport charges.
Can't see that capacity for 3 weekly flights or so would be a problem. If capacity wasn't a problem for summer 2010 why would it be in 2011 when Krakow have lost a number of routes and Ryanair are dropping Weeze and adding Cagliari.