Because they played significant time in an easy era to score in.
When they're era adjusted, Andreychuk is nowhere near Alfredsson offensively. Alfredsson destroys him in points and PPG. Andreychuk's goalscoring rate is slightly ahead, but not particularly impressive compared to Alfredsson considering the fact that Andreychuk had no other use on the ice except to score goals. Meanwhile, Alfredsson did everything on the ice except play goalie.
You might think that the old school guys are easily fooled by the high scoring 1980's, but they were also around back then to watch Andreychuk in his prime. He was a 1-dimensional, cherry picker who scored goals when it was easy to score while getting lots of ice time on terrible teams. Maybe some of them ask themselves if they could've won if he was their best or 2nd best forward.
Andreychuk finished Top-10 in G (2), Pts (1), PPG a total of 3 times.
Never finished Top-10 for a Trophy. Never finished on a AS Team.
Alfredsson finished Top-10 in G (3), Pts (3), PPG (3) a total of 9 times.
Won the Calder, Clancy and Messier. 2nd AS Team. Alfredsson destroyed both Andreychuk and Mogilny in Trophy voting throughout their careers.
Andreychuk was never anywhere near the offensive or defensive player that Alfredsson was, not even close. Andreychuk actually adjusts to 11 points fewer than Alfredsson in 400 more career games.
Mogilny is at least the same class of offensive player.
Mogilny has the two big goalscoring seasons, but his production dropped off otherwise due to injury and inconsistency. He doesn't have nearly as many productive seasons as Alfredsson and Alfie was much better defensively too. Mogilny just doesn't have a high enough peak to contend with Alfredsson's consistency or all round game.
Re: Mogilny's PPG... his career effectively ended at 34yo and he played 5 years before the DPE ever started. His unadjusted PPG is 1.04 at 990 games (38% of those games pre-dead puck). Alfredsson rolled back to 1002 games is at 0.99 PPG (100% of those games played after the start of the DPE). Era-adjusted, it's 1.09 Alfie to Mogilny's 1.06.
They had fewer points combined in their Stanley Cup winning postseasons than Alfredsson posted in his SCF's season.
I think that the voters have adjusted quite well to the bigger league / less chance at a Cup (Sundin, Oates, Housley, etc). The Cup is a nice cap to your career, but it's not the be all and end all.
This should also be where we point out that neither of Andreychuk or Mogilny produced as well as Alfredsson did in the playoffs... which really sucks because 1/3 of Mogilny's and well over 1/2 of Andreychuk's playoff games occurred between 1980-1995. They should have posted video game totals, not less than a player whose career started at the beginning of the DPE.
I think it's quite the opposite with those two players, there's a vocal crowd who believes that they should never get in. Andreychuk has some backers, I don't think Mogilny has any.