Rose tinted spectacles, old codgers wanting to re live their youth, stuff becomming rare because we chopped them up and chucked into a skip, or simply wore them out and chucked them away.
Dont doubt there are lot of factors at work here, and different folk have differing reasons for wanting a particular bike.
Take my H2 - they seem to be "sought after" / "collectors" pieces, but why? At the end of the day it is just a basic piston ported 2 stroke, that neither handles or stops particularly well, and only has mediocre performance compared to a modern bike, and has pretty poor fuel economy.
Jeez - now I am wondering why I like it