Strictly my opinion ...
He was the best player on the ice practically every game he ever played, so the holes in his game that became obvious in the NHL - consistent effort, defensive awareness, playing without the puck - were never addressed. One wonders if the first freshman Hobey Baker winner since Paul Kariya was ever on the ice with a coach telling him "okay, Jack, we're going to work on your defensive game today".
Because the game has always been so easy for him, he's easily, and visibly, frustrated when a play doesn't work. Self-motivated players instantly adopt a defensive posture. Jack tends to hesitate before reacting; his body language betrays his frustration. There is some emotional immaturity involved here as well.
On the other hand, he can backcheck like a demon, easily chasing down an opponent's rush with his incredible speed. We've seen him do it many times, but too often we've seen him coast and watch in similar situations. Eichel deserves to be labeled "lazy" and "apathetic" for plays like that. Moreover, what lesser teammate wants to follow that example of "leadership"?
What he needs is a more experienced, highly skilled player as a role model, and/or a coach who will give him the tough love he needs to correct his flaws. Unfortunately, he's so skilled that everybody on the team tends to defer to him. That doesn't help fix problems that were apparent early in his rookie season two years ago.
It's a shame. This kid is by far the best player on the team, but he could be so much better, as a player and a leader.