While Stanley Morgan certainly deserves consideration for the Pro Football Hall of Fame, I can understand why he is not enshrined in Canton.
To me the best way to compare a player statistically is not from one era to another, but instead against his peers. The game has changed quite a bit, so certain statistics from Morgan's era do not accurately relate to today's game. It's like comparing passing completion percentage or yards passing from thirty years ago to today's quarterbacks; it's almost useless.
In Morgan's favor:
As pointed out above, at the time Morgan retired he ranked 5th in career receiving yards, and since he retired only one player has more yards per reception in his career. 10,000+ receiving yards is still a pretty big deal today - only 40 players have reached that mark - but back then that was almost unheard of. Morgan led the league in yards per catch three straight years, and over a five year stretch was among the top four every year: three firsts, a second, and a fourth. All this was accomplished with a team that pounded the rock and with a quarterback that was not considered to be elite in terms of touch and passing accuracy.
Working against Morgan:
It's not fair, but many voters still look at championships - and Morgan has no rings. It makes no sense since the HoF is an individual award and a championship is a result of an entire team playing better than the competition, but unfortunately this factors into voter's decisions.
Personally I like to look at how a player performed in comparison to his peers, to see how dominant he was at that position during his career. Although the figures above work in Morgan's favor, if you look at his year by year stats it shows he wasn't really head and shoulders above other receivers of his time. Here are the number of times he was in the top ten for key receiver stats in his career:
Receptions: just one, when he was 4th in '86
Touchdown receptions: (2) - 1st in '79, 10th in '86
Receiving Yards: (3) - 2nd once, and 9th twice
Yards per Catch: (7) - 1st 3 times, 2nd, 4th, 6th and 10th once
Other than the yards per catch, that's a lot of times that he did not rank in the top ten in key WR categories; that makes it tough for him to get in.