He's been playing with terrible players all season without any PP time, averaging only 10 minutes per game. It's just not fair to look at his stats.
Reasons he is ranked high:
-Good size(6'1) and lots of room to bulk up.
-One of the fastest skaters in the draft.
-Plays a mature game already and positions himself well on the ice
-Rarely makes mistakes
-Can play every forward position.
-Very slick hands and elite puck handling ability.
-Good wrist shot.
Good report here:
Rasmus Kupari - NHL entry draft 2018 prospect report October 2017 | Pucks for Breakfast