Because he was playing in the AHL at 18, 19, and 20 years old, and wasn't ready for it? Because Buffalo is terrible at developing players? Because Buffalo is a terribly run organization? Because he had some injuries?
He played 65 games as an 18 year old and had 28 points. (35 point pace)
He played 51 games as a 19 year old, and had 27 points. One less point in 14 less games. (43 point pace)
He played 49 games as a 20 year old, and had 31 points in 49 games. Four more points in 2 less games.(51 point pace)
He also played 12 games for Buffalo last season, where they said he played well in his call-up, and had 4 points in 12 games.
It sure looks like, when healthy, he is progressively getting better, year to year. At least based upon the stats.