I think it is a combination of factors. Carter was a great WR for his time, but his stats are less impressive as the years go on. NFL rules have favored the pass - I think there were almost 20 guys this year alone that surpassed 1,000 yards for the season. You also need 80% of the vote to get in. Sounds like some of the WR votes went to Tim Brown and Andre Reed, so I can see how Carter might have hit the majority vote, but still missed the 80% cutoff.
Looking at the guys who did make the cut, I'd be hard pressed to say Carter should have made it in instead of them: Defensive end Richard Dent, running back Marshall Faulk, linebackers Chris Hanburger and Les Richter, cornerback Deion Sanders, and tight end Shannon Sharpe. Faulk, Sanders, and Sharpe defined their respective positions for the modern age - I'm not sure you can say the same about Carter. He also doesn't have any Super Bowls under his belt like most of the others.
Very impressive #2 WR, but not a game changer like these guys were. Here's an interesting table to consider. I saw online that someone had totaled the rankings for Brown, Reed, and Carter in terms of the # of times they ranked within the top ___ WRs over their career:
| ||Top 3||Top 5||Top 8||Top 12||Top 20|
Carter made the top 12 in most of the years he played, but he was only a top 5 WR one year and never made the top 3 at his position. I haven't followed Carter's career that closely, but is there something other than being a local hero that qualifies him above these guys or the guys who were just elected?