I don't even remember that as being a particularly notable SNES game. I played it, but only once, and I remember that cattle prod that you have as your weapon or something being pretty crappy. It also had those crummy Wolfenstein-era first-person sections. So really I have no idea why someone would buy a zillion copies of it.
I've seen two other things like that: there's a Donnie Darko special feature about a guy who collects all the DD stuff and is the biggest DD fan and has like 30 copies of the regular DVD laying around, and my old local used-video-game store, which was selling copies of Super Mario Bros./Duck Hunt for like 10 cents apiece and was using them to hold up table legs and stuff.