I have a hard time putting Bettis on the "in" list. He's 5th all-time in yards, but as others have used the term he is a "compiler." He wasn't really all that special when comparing him to his contemporaries at RB, he basically got a bunch of carries which is why he ranks so high in rushing yards. He's a guy who had his career prolonged by become a part-time back with short yardage responsibilities. He never led the league in rushing and his lifetime average of 71 yards per game isn't really HOF material for a guy who would have to get in based on his rushing totals since he wasn't much of a reciever. I would put Tiki ahead of him because of how really good Tiki has been the past couple of years, Bettis was never this good.
On Bettis as a compiler, one of the best things in his favor is his 2 All Pro selections. I posted on this earlier. That means he was judged to be one of the top 2 RBs in football twice. As a comparison, Tiki has achieved that only once. People tend to knock Pro Bowl selections, but he had 6, which is a pretty high number for a RB. Bettis also is a "big back", and some might give him some level of credit for that, given that he lasted for so long despite the pounding incurred by his style. And, finally, he will be seen as a key leader on a Super Bowl winner.