More hassle than it is worth, has to be part of the reason.
As others have said already there are plenty of examples in the rest of Europe where you can go land and park for some time on an airport not dissimilar to Luton/Stansted without breaking the bank.
Sure enough the pressures of traffic at Luton/Stansted at times is such that it would be inappropriate/impossible to merge light GA traffic but if they wanted they could accomodate small traffic a lot of the time.
So face it guys, we are an unwanted breed.
FD