First, take out the projected 2014 draft because its never smart to allow a future draft to influence this one. Nobody knew McLemore's name before this season and he's widely considered the #1 prospect, and the best big prospect is out for the next 6-8 months with torn ligaments. We have no idea how things will turn out.
Back to Zeller, he does project to be a solid scorer, but there are a number of things working against him. He doesn't have the mass and (more importantly) the length needed to project to a high rate of efficient scoring in the NBA. He also hasn't improved as much on a year over year basis as you would expect from a guy in his second year. One other thing is that he doesn't quite have the star power as others... Though that's much more subjective.
Most importantly, it's a case of people simply not thinking he's quite as talented as some other guys.