Because of the 1-2 hour ahead time difference in Central and Eastern Europe, Wizzair have no need to base aircraft at Luton or other UK airport. Just like easyJet and Ryanair, they aim to get all first flights of the day off the ground from their home bases around 5.30am-7am. So by the time the flights get to the UK with the time difference, they are only slightly behind UK based aircraft, leaving from there bases...
One of the keys to Wizzair's success is its extremely low cost base. Its a fact labour costs and airport fees in the UK are far higher than the likes of Bulgaria and Romania.
If Wizzair were to base aircraft in the UK or more to the point LTN, where would they fly to? As I said above, they have no benefit of flying to their current destination with aircraft based here.
Ryanair and easyJet virtually have the monopoly on all the popular destinations from the UK to western/southern Europe. Why would a lessor known brand in the UK want to go up against them, when prices and yields are rock bottom at it is.
The vast majority of the passengers flying Wizzair to and from the UK is immigrants, where the airline is far more well known in that country than here in the UK.