No set criteria list. He just has never got enough votes by forum members when it counts.
Long time forumite here @mrchallenge started this off back in 2009-10 and we’ve been voting every year since then. Including this year when Mark Damon finally made it in. The process has changed a little over the years but is always effectively an open poll done via the forum. Here’s the link to the relevant thread: SWDb Spaghetti Western Hall of Fame - #37 by mrchallenge