It has been touched on already, but a few things are keeping Dez lower on the stats sheet.
One being the lost year with Romo.
Two being the next year with Dak learning as he went. They connected more at the end of the year, but I think the first 10 or so games were growing pains for this duo.
Three, and probably most importantly, is that the offense wants to/can run the ball compared to other teams. Not only is the receiving corps crowded, it's not the preferred avenue of offense.
That being said, I still don't think he has QUITE lived up to the contract. But by no means is this a bust or wrong move. Let's not forget that just having him on the field and knowing what he can do makes defenses pay attention to him.