I think it’s down to a multitude of factors:

1) The market for these cars isn’t exactly huge to start with. Only car enthusiast really know about them, the “average” car buying isn’t really aware of them.
2) With the state of fuel prices the “average” car buying isn’t in the market for any V8 let alone these, shrinking the market further.
3) With what’s happened over the last 2 years maybe more people are unintentionally holding onto these cars as a “collector car”. Me for example used the car as a daily 2 years ago, now it comes out once a week (maybe) and stays in the garage am I’m taking more care of it. So I’m less likely to sell and get another car and more likely to hold on to it.

Thats my 2 pence worth anyway