Mack contributes to a higher percentage of his teams goals, and just outscored Kucherov while every other member of the Avs top 6 missed 10+ games with injuries. He also outscored 2nd place on the Avs by 43 points.
Kucherov, on the other hand, has scored 24 more points (-2 g, +26 a) since 2017-18, but his team also has 103 more goals over that stretch, which is a big reason why he has 26 more assists. And, even in his 128 point season, he only outscored 2nd place by 30 points (and 3rd by 36 points).
So, other than having more assists as a function of playing on a better team, why should this be Kucherov?