For the same reason that most high end young kids in juniors are "centers". When you're young and very good (an above average player (or better) among your peers), your coach will want you on the ice as much as possible, playing the most important position possible to have the largest impact as possible on the game. Which typically means either as a center or as a D. Not everyone moves to those spots, as sometimes the team already has players for those roles. Other times coaches will move players around (D-F, F-D, etc) to see what 'suits' a player. Sometimes despite being able to produce offensively, they still do not create offense the same way as a center/winger might. Look at Muzzin for example. He's not someone I'd describe as an "offensive D" (compared to say Ghost, Yandle, etc), yet he's producing very well and is on pace for 50 pts. On the flip side, Drouin is on pace for 53pts, yet people would use words like 'dynamic' and highly 'offensive' to describe him (and they wouldn't be wrong).
For me, I played D in soccer because I wasn't highly offensive. I could read and predict plays extremely well, and I was fast. But my game was always very defensive minded, and as I got older and played in more competitive leagues, playing there was more and more natural to my style of play.
So while sometimes I think it is simply "Johnson you're playing D"... while other times the coach can see how a player plays, and will try different things to see what works and what doesn't. I mean Brett Burns was drafted as a winger, and was only placed on the blueline once he turned pro. Hard to imagine that now after a Norris win as the best D in the league. I imagine that size and skating play a part in this with regards to hockey.