There are several good reasons why, but so far they haven’t been good enough ...
Pressing: For Solskjær, active press is even more important in the front than in the middle. Pogba is not a good presser, he is heavy and it probably costs him more energy to press actively than others, and he won’t exhaust himself as he wants to be sharp enough when in possesion. Cantona was a bad presser as well, it’s not so much laziness as priorities. But ... so far, our collective press is a work in progress, which is normal. It takes time to drill a good collective high press, by now it’s better, but only so-so. Medium height press we are quite good at, which is why few teams have troubled us much of the time barring Chelsea’s opening minutes.
Transition to attack: Pogba, on paper, should be a perfect player to transition from defense to attack quickly, particularily against high pressing teams. He’s strong and technical to shield the ball, and has fantastic long passing. If he plays well, most teams won’t dare high press us. But ... so far, in practice, he isn’t succeeding. When he is crowded, he should play quick, simple passes, but instead he dallies and loses the ball in dangerous situations when the other players are storming forward. When he has some space, to often he is using crucial time looking for the long through ball on goal, when he should rather play it simple and faster through the lines and set us up in 4 vs 4 situations repeatedly. Can he learn this? Solskjær and his team is certainly hoping so, but he does seem like a real slow learner when it comes to tactics.
Space: Pogba needs more space than a lot of no 10s. He’s quick on the first yard because of his stride, slower in accelerating on yards 2-10 because of his heavy build, but very fast again when he gets up to speed. This makes hin good to lose a man for a pass, but not so good at losing a marker or making or losing a man for a short run around the box, but again, very good at losing a man at a counter or when approaching the box at speed from behind. He has never to me been particularily useful as a no 10. He can be from midfield. But ... so far, our front four have not been good at keeping the ball in transition. The balance between direct attack and keeping the ball often enough to get second and third waves of attacks is difficult, and we’re struggling with that. Particularily Rashford goes for the kill every time, and either makes it or we lose the ball. So whemn Pogba runs from deep we’ll either be far a head or have lost the ball. Can we improve this? Yes, certainly, but it takes both practice and good enough players able to learn. And confident. And patient.
Most of this worked well in preseason, implying that we are in a good development, but it’s much harder at PL 100% tempo to pull of, showing we have a way to go. But to a degree I think it’s right to say that Pogba’s problems are partly due to how the team works, and partly due to his own need to develop tactical intelligence.
A last note on the 4-3-3 with a left-sided Pogba: Yes, that was his best attacking games last season, but ... he left a hole behind on the left each time we lost the ball, and it impeded our left sided forward, where we have our most dangerous forwards. And it meqnt having only three players pressing from the front, which made it easier to force us low in the field, as even Newcastle managed even when we were fresh and good.
We are not Klopperpool, and will never be better than the second best at Kloppball, but either way: Is it even certain Pogba would be a starter at ‘Pool currently? In which position?