Depending on where you are, could they have been used as temporary accomodation after last years floods?
In which case the interiors would be a bit tired, having had the equivalent of about 5 years use, but structurally they should still be as new.
Or is it Spring fever? Everybody suddenly deciding to get a new van for the start of the new season?
Or is everybody in your area suddenly getting too old to cope with a van?
Somehow I doubt if it's peoples finances. Once you've got a van it remains a cheap way of holidaying. It's when you come to change it that you give up because of the cost of a new van.