Exactly the NBA drafts on potential and when JMM got in the game his freshman year he was productive.
Here is the thing- he wasnt.
Everybody remembers the ACC tourney when he stepped in for Henson and looked like a seamless fit on that good team. And thats what all the pro hype was about- he looked really good after finally getting a chance. The problem with that line of thinking is that he got plenty of playing time throughout the season- and was complete garbage for most of it.
But when he made the decision to come back, his stock was still at its peak following his good finish to the year. And since he didnt declare for the draft, scouts never bothered scrutinizing his earlier games. Superficially he looked like a great prospect- big high school player, pro body, athletic, some big games against good teams. But realistically he was a guy who played badly for 90% of the season, is awfully short for an NBA PF, and cant play post defense.
I just think his stock would have dropped as draft time approached