Because he lacks many of the skills that are associated with wings, namely the ability to handle the ball and create for himself and others off of the bounce. He's also slow defensively--which doesn't have anything to do with his athleticism, but rather him being reactive which makes him a half step slower when trying to defend.
AWIII is a fine shooter. There is a place in the NBA for catch and shoot guys who can make jump shots. And he's got the requisite size. But anyone who expected him to be anything more than that was engaged in wishful thinking. He's a terrific player, he was exactly what we needed to fill Mali's departed shoes, and he's going to likely lead us in scoring. He was also the difference between this team being a borderline top 25 team or a team that could be top 5 / legit national championship contending team. But too much has been made of him getting the opportunity to "audition" for the NBA. At this stage of his college career--five years in--he is what he is -- which is a player in the Wes / DNic mold.
