Because his footballing brain for the CM position is far ahead any normal CM let alone a Pogba. His intuition for that role was that great, he was trusted to play there as a 16yr old for Arsenal in their prime, near the Invincible era. This was a guy capable of emulating Paul Scholes. Even a washed up version of him, made a far superior CM than Pogba right now. Aside from his brain, he naturally picked up great pockets of space in front of the defence, he naturally knew how to position himself off the ball, again another player who was a pass-meister and therefore he could stay deep but spray the ball around and control the game, the dribbling aspect of his game was minimal and thus less likely to be caught out of position. If you compare Iniesta/Coutinho to Scholes/Xavi.. you'll notice that the first pair, dribble alot with the ball, they probe at defences and play the killer pass, and they roam around, ball-carrying as well as passing. Whilst the latter duo and say Pirlo, even though all three are very nimble on the ball and technically superb, they don't ball-carry. They tend to stand still and pass the ball around, acting as a hub and therefore if there is the occasional loss in possession, they're still plugging a gap and not leaving a gaping hole. If you played Iniesta as a CM, it would be a problem in the long term because he'll be out of position as he likes to dribble alot and go forwards with the ball. Someone like Modric is a rare blend of the two, but again his bursts tend to be in the CM area and not as much down the flanks or in the final third, and he's so nippy and dynamic, he gets back again into a good position. Pogba falls into the Iniesta/Coutinho school of heavily relying on his dribbling and he can't just sit still and spray the ball, it is counter-intuitive to his natural way of playing the game and that makes him a bad fit for a CM. Just like in reverse, it made Fabregas a bad fit for a Barcelona attacking mid, because his dribbling was very average but passing is his game and therefore his best years in his career have tended to be when he's operating as a CM.