That's because he was that kind of player in the youth ranks. Moyes and the coaching set up tried to turn him into a Giggs type player. His decision making from that point on suffered. He became a 1 trick pony which was to dribble, and he didn't have the pace for that.
Anyone that watched him in the youth ranks, would liken him to a Zidane type playstyle (No not as skilled or as much potential or anything resembling the greatness of Zidane. Just the playstyle). Players of that ilk that although slow, made great decisions, and used their great skills to tip the scale towards their side's favor. I remember him playing as a striker, and all he did was use his great first touch and ability to bring others into play that allowed United to beat Spurs in the u21 finals.
It's a shame that United tried to change his natural play style, and along with giving him the pressure of being a young star, allowed his attitude to change from what used to be praised as a hardworking quite boy. It's partly his fault of course, but I don't think he was done any favors.
Another early performance from him in the League cup.
Great through ball from januzaj against Spurs. Kind of passes he was capable of at youth level.