Main factor to me is that Gretzky always played on a stacked team & Mario didn't.
That point always comes up... but the 'Mario camp' is putting WAAAAY too much significance on it. Gretzky, no matter what team he played for - with or without bona fide superstars - was always able to rack up sizeable points, while at the same time making those around him that much better. His LA/StL/NY team rosters were no different in overall 'stardom' than half the other teams in the league at the time, and also including the Pittsburgh teams Mario had to play for early in his career.
Gretzky was almost 29yrs old when he joined the Kings in 1988. Even at that age and above (with arguably a hockey player's point-producing prime years behind him) he was still putting up very very impressive numbers with teams that had little to average depth, year-in, year out, until the end of his career. Therefore, for more than half his career (11 years out of 20), he put up 42% of his total career points with those teams. Just like the Lemieux fans believe if Mario played more games with a better roster, he'd have more points (which is true), a Gretzky fan can most certainly believe since Gretzky could put up such great numbers even past his prime, if Gretzky played his first 9 years of the NHL with mediocre teams, he would still put up earth-shattering numbers, and those total points would be much much higher than even the impressive numbers he accumulated when he joined those said teams at a later age. People tend to forget that when the Oilers came into the NHL, they were a bad team (standings-wise) and stayed that way for a couple years. Seeing what he did with a bad team in 1979 coming into stardom, and seeing what he did with a bad team in 1988 in LA at 29yrs old – it’s hard to argue that he wouldn’t have achieved the same success with any bad team in the 1980’s. He’s a prodigy!
Lemieux, on the other hand, during his 19yr old-30yr old 'prime' playing age (which accounts to 9 seasons of 60 or more games) put up good-great numbers with a lacklustre roster for the first 3-4 years...BUT he didn't excel in that scenario like Gretzky did during his lacklustre roster years (1979-80 and 1980-81; and LA onward). And when the Penguin's roster started to blossom to the point where he was playing with a star-studded roster for the remainder of those 'prime' years, Lemieux still couldn't excel like Gretzky did during his star-studded rosters of 1981-82 to 1987-88.
Hence the difference in numbers and player greatness. Yes, Lemieux played fewer games in those prime years, but Mario's game playing numbers are still well within the appropriate sample size to make equal comparisons. And I understand people make a fuss about the dead-puck era or lower scoring as the decade went on (and that affected Mario’s point-getting), but they were only separated by 5 years and I don’t think there was enough difference in the game between 1980-1999 to classify Gretzky and Leimieux playing in different eras. If you take out the ‘abnormal’ Gretzky and Lemieux from the scoring stats, the next superstar in the point standings gave you totals that were in the 120’s-130’s, with the exception or two. So, the playing field was more or less equal for both Gretzky and Lemieux. Any advantage for either of them playing 5 years before/5 years after the other’s arrival in the NHL, is slight, in overall comparisons, in my opinion.