Essentially because as a Porto follower and what I saw of him at the time I think the role he was assigned (which couldn't be any different, mind, hence I think it was the wrong club for him) doesn't capitalize on what were his greatest assets and forced him to "recondition" himself for the greater good of the team.
Loads of talent yes, but when that talent never showed in the form of great tactical awareness, an above average long-passing range or tackling, I don't see how he can perform very well in CM, let alone a 2-man CM as United usually plays.
And a player that is a first team player in Porto at the age of 17 can never be regarded as a late bloomer. More the case of a natural path that was diverted and hence slowed down.