a couple of other reasons are specific to the players involved
Rodney Harrison was HoF performer on the field. But his peers viewed him as over-the-top instense and even "dirty". This kept him being snubbed from pro bowls and all-pro teams over and over and over again. HoF voters count up pro bowls and all-pros.
Slot receivers have never gotten their due and were always overlooked until the Belichick version of Perkins-Erhardt elevated it with guys like Troy Bown, Wes Welker, and Julian Edelman.
Big Vince is a HoF-er in production, but as a space eater in a two gap system he never had the sack s and pressures of other interior linemen, He ought to get in, though.
Ricard Seymour ought to make it, too, but he gets overlooked maybe becuase of his too-soon departure form NE.