It's because he runs a horribly boring dink and dunk offense. In the games I've watched the ball rarely travels more than 5-10 yards vertically in the air. The YAC is all that keeps his YPA acceptable.

If you watch his college highlights, it's stunning how often he was throwing to guys running free. I've never seen him deliver balls consistently and accurately down the field in tight windows, which in today's NFL is pretty much mandatory if you want to be among the top teams. Right now, the Titans seem to be content with him being a game manager. I think you need to ask and get more out of a guy you made the #2 overall pick unless you're content with 10-6 and maybe a wild card win being your ceiling.