I think the number one reason is the fact that more were sold.  Number two is
that when these vans get higher miles on them people either do the head gasket
thing and then want to sell the vans immediately (why they want to sell a van
they just sunk $1500 into is beyond me but this is usually the story) or fear
doing the head gasket thing so they want to sell it real quick before that
happens.  True there are some low mileage older Westies out there but the
problem here is that I think that the heads go bad when the vans are not being
driven too (maybe even faster since the coolant isn't circulating).
To me a higher mileage westie that has had the gaskets replaced and is being
driven every day or very frequently is better (motor wise) than a low mileage
one that sits all winter and then is driven a month in the summer. Sitting is
not healthy for a motor.
Anyways the newer westies are considered more desireable since they could have
low mileage without the problems associated with long term storage, they have
a 2.1l motor instead of the 1.9l (there is a difference) and usually come
stock with power steering, A/C, and have power options that the earlier ones
didn't.  Therefore people tend to hang on to them once they have purchased
them.
Thats my two cents.
Ken Wilford
John 3:16