Because he's not a dm, and is shit. I've watched Juve A LOT these past couple of years and he's not had one good performance, and doesn't play as the deepest midfielder. When Arthur has been out, their midfield has gone to absolute pot because none of them are good enough.
He was even moved to left midfield because he was so useless in the centre.
Which is actually the issue. Rabiot is a DM, he has always been and that's where he has played well and shown potential. Now it is known that he didn't want to be a DM, there is even an interview of Thiago Motta who explained that he needed some convincing.which was a shame because it was his best role.