Yeah, it's a good thread topic. He's a curious player in terms of his development. I can't think of another player right now who had the really good rookie year, three fairly disappointing years, and then suddenly surged back at superstar level in year 5 (and beyond) as he has done.
The Avs were a better team in his rookie year and from his year 5 (and worse in between), so there is that. But I don't think that alone explains it because in his year 2, the Avs were still pretty good (.500-ish) yet he fell of so sharply it was like two different players.
It can't be entirely explained by ice-time / opportunity either, as it was his fourth season when he really became the top center on his club... and yet he still had only 53 points (though that was enough to lead the club, so horrid were the 16-17 Avs).
The pivot season here is 2016-17, though, because that's right before he hit superstar level and it's the season he became the top center/player on his team. Unfortunately, the Avs were a train-wreck that season. Perhaps if the Avs had been at least more competitive that year, MacKinnon would have put up, say, 75 points instead of 53. And then we'd see more of a gradual increase from his third to fourth to fifth seasons. But since they sucked very large bone that year, his stats aren't very impressive even though he led the team.
And then, the big break-out in year five.
Besides his own psychology and aroma-therapy or whatever he's using, it might also be a thing where he needed to physically mature and he needed to be the TOP DOG on his club. When he was younger, he was always behind Duchene or Iginla or Landeskog. Some guys really respond when they are suddenly elevated to a leadership / top-star role on a club (and others wither and wilt).