For whatever reason his jumper from high school never ever developed, not even really a little bit. Zeller is a good screener and pretty good rebounder. His defensive stats are pretty good for a big man that doesn't get a lot of blocks.
If he could have had some sort of jumpshot out to 15-20', I think he would have had a different career. As it is, dude has still made a bunch of money and will stick around the league for many years to come.