It could have to do with the fact that it does tend to be a neglected position.
Kids don't grow up wanting to be a punter. There aren't NFL Channel programs on the Top 10 Greatest Punters. Therefore, people tend to not be steeped in the arcana of punting lore.
As to many punters surpassing him on raw numbers, I think the debate is whether he was historically important for revolutionizing his position. I don't know enough to make that argument (see above). Do you know enough about the history of the position to say he definitely wasn't and didn't?
A criticism of the later players surpassing an older player in raw numbers being grounds for exclusion argument could be along the following lines. Let's say Jesse Owens wasn't in the Track and Field Hall of Fame, and advocates were thinking about inducting him. A detractor brings up the fact that there have been many faster sprinters since that have surpassed his times. Would you think this a good argument to exclude him?
Somebody said it is easy to quantify punting. What if you are on the opposition 45 yard line, punt it out at the 10, that is a 35 yard punt included in your average. Bad play by the numbers, right?