Chad Ford with ESPN said he would have been a top 5 lottery pick last year but now is probably a fringe lottery pick. He said Zeller didn't improve as expected this year. Another thing, though, was people started picking apart and finding negatives in his game. This is the risk kids take when coming back and not leaving when they are hot.
Plus, another reason it is better for him to leave this year is that this class is very weak. It's one of the weakest draft in years. Next year will be loaded, though. Definitely a better business decision to leave now.