Yes, he was a big reason that Denver won those Super Bowls, but he barely played. When he got hurt in 1999, Olandis Gary stepped in and ran for almost 1,200 yards in 12 games. When Davis got hurt in 2000, Mike Anderson stepped in and ran for almost 1,500 yards as a rookie 7th round pick despite only starting 12 games.
Denver's offense produced those running backs. Gary and Anderson never did anything else outside of Denver.
Davis had a couple elite seasons, but his place in the Pro Football Hall of Fame just vastly lowered the standards for running backs entering the hall.
Does Chris Johnson get in now? He had a 2,000+ yard season. Or how about Priest Holmes? He'd have put up monster numbers if he didn't get hurt. In fact, in looking at the numbers, there is no way you can justify keeping Priest out with Davis in.
Meanwhile, a guy like Watters had sustained success over a decade and isn't in. Makes no sense other than Davis is on TV now and Watters isn't.