They are reaching an age where they are only worth anything to people who are interested and prepared to work on them as they are all getting a bit tired I think, for that reason I suspect everyone who isn't interested has decided to sell up and get a more modern, and in their opinion hopefully more reliable car, thereby swamping the 2nd market with hundreds of perfectly good cars.
The newest mk3 will be 13 years old now which is very old to a lot of people. Seeing anything pre the 'new' reg layout '**02 ***' is getting less and less common as people generally want newer cars I think and are flogging the old off at next to nothing or scrap value.