I guess he never won a championship or defined a team in a way players like Lawrence Taylor did. He spent most of his time with the Rams, but they weren't much during his hey day. I'm guessing he was underrated during his career, as well. He only went to the pro bowl 5 times, despite 10 seasons with 10 or more sacks.
Yeah, It's crazy to me how forgotten he is in an all-time capacity as a pass rusher. I can't really figure it out. It's not like he's a quiet, unassuming guy. He's had success beyond his playing career (Clay Matthews' position coach), and people seem to speak highly of him on a personal level. Has he alienated himself in some other way? If only Reggie White and Bruce Smith have more sacks than you, you're not exactly chump change.
It must be that the era he played in was flooded with great DE/OLBs who had long, remarkable careers (B. Smith, R. White, C. Doleman, L. Taylor, D. Thomas, etc). Anyway, if he can't seem to get a sniff of the HOF, it's hard to imagine any DE or OLBs from this current era deserving it any more. Nobody's even close, unless DeMarcus Ware can keep it up for another 6-7 years.