Because they were so ubiquitous, they weren't desired. As they get "older" and have that "old" "vintage" thing worked in, and there's fewer of them- they'll be more desired- if just as old fashioned department store girl bikes are. As old things people used to ride in the 80s listening to their Wingers, Motley Crues, and Color Me Badd.

IMO, those old MTB groups are the cat's pyjamas.

Old 730-735 XT never goes out of style.