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
I don’t see any new Lead Actors since 2017
What Category is Mark Damon in
Don’t See Him in Lead or Supporting
How many votes is needed
Sorry just don’t get how Craig Hill Isn’t In It
Probably Deserves it over 2 or 3 Already On It
Damon was voted in this year but his “poster” and bio are yet to be added.
We only vote one person new in each year now and they can be from any of those categories so no guarantee there will be a new lead actor every year. Robert Hundar was last year’s inductee
I Get That The SW is Done/Over and That There is a Limited Amount of Choices
But Only 1 Per Year Seems Silly
Craig Hill > Mark Damon (Definitely IMO 
)
I Vote We increase The Number of Inductees Each Year To 4
Those 4 Can Be From Any Category
What Does the Rest of the Board Think?
Well, we used to induct more per year but pulled the number back in to ensure we still had worthy candidates each year. If we voted in 4 new inductees every year we’d be down to Sean Flynn and Gianni Crea before you know it. 
4 Inductees Spread Out Over 9 Categories
Yes You Will Run Out Eventually I Understand That
But At One A Year I’ll Be Dead Long Before That
Again I Vote we Increase The Yearly Inductees to 4
Who Else Votes With Me!?
Joe
Calm down, mate. 
Totally Calm Here No Worries 