There were never that many around, really. Even in the mid 1980s when KTM and Husky were well established (and making very good bikes), they actually only sold tiny numbers of them.
I was told that my old '88 KTM125GS was one of ten or twelve brought into the country. Obviously there would have been more 250s out there.... but even if there was ten times as many, its still not a lot of bikes. After 20+ years, it makes sense that you hardly ever see them for sale.
Straight off the top of my head, I can think of about half a dozen pre-85, Euro enduro bikes that are sitting around doing sod-all and are not for sale. Their owners are collectors/Vinduro-ists, or they have sentimental attachment to the particular bike in question.
In the older era bikes, a lot stuff was converted to VMX spec.