During the naming of Olin Kreutz to the 2001-2010 All-Bears Team
The biggest single reason, I think, is because offensive linemen are harder to quantify statistically, especially with a mainstream number, than a receiver, running back, quarterback, or any defensive player. Quarterbacks probably have the most quantification - yards, touchdowns, interceptions, touchdown/interception ratio, quarterback rating, completions, attempts, TD%, INT%, completion percentage, rushing statistics, in some cases receiving statistics. Even for a defender you have tackles, assists, tackles for loss, sacks, passes defended, interceptions, fumbles forced/recovered. What does an offensive lineman have in the mainstream? Sure there's pressures, hits and sacks allowed, but usually those are applied to the entire line. There isn't really a mainstream individual offensive line statistic. (And since when at the Hall of Fame would they ever use Pro Football Focus's advance metrics?)
So that leaves the only offensive line metric being "What did you see on film." Once a player retires, there's only two things left of his career - his statistics/accomplishments and his film or "what do you remember of him." And since an offensive lineman doesn't have one of those (statistics), that leaves his film.
Which of these will be more remembered: that Receiver A retires with 12,950 yards and 140 touchdowns, and maybe two Super Bowl Rings, or Offensive Lineman A retires after starting at his position for twelve consecutive years, misses only one start, and while playing develops a reputation for being one of the best blockers of his time. Pretty sure, you'll be zeroing in towards the receiver's numbers. Time in the NFL is such that by the time Offensive Lineman A is eligible for the Hall of Fame, no one remembers, not while this guy that "just reached #2 All-Time in Receiving!" is coming eligible.
That's another thing statistics allows... an attempt at career perspective and ranking. We know the rushing list - Emmitt is ranked #1, Payton #2, and we know Jerry Rice is the #1 receiver all-time - but we don't have any such ranking for an offensive lineman.
So that's it, I think. The biggest reason why there aren't more linemen in the Hall of Fame is because there's no mainstream individual metric for offensive linemen to put them in any kind of perspective order.
What do you guys think? Anything I'm missing? Sound off!