Maybe because he hasn't shown that kind of skillset yet? Too easy of an answer? I mean, I am all up for trying him again in a deeper role, but I personally am very skeptical about him being a big contributor in this position. He came to us as system player in a total foetball style. Short passes, positional play, movement without the ball, distances between players stay consistent, patient possession game. Donny excelled in being a player with a good feel for finding space to either be passed to or get on the end of something. He has also been a formidable striker of the ball. He has played deeper for Ajax. But when he excelled then often in a Thomas-Müller-type role as attacking multiplicator.
Can he play somewhere else on the pitch? Of course he can, Ronaldo would also do a job as fullback I guess. The question is, what do we get if we transform Donny into a slightly above average CM when he has the skills to excell in a posession based system like Bayern or Barcelona right now? We are very far away from such a system, we celebrate Bruno, Pogba, Rashford and Greenwood for fast attacks and quick transitions. Thats the opposite of the system Donny is used to. Plus you want him to change his position to be more allround. Donny will have to learn all the things connected to playing as a CM, tackling, intercepting, spacial awareness - is he capable of doing that? Maybe. On the top-level? Wouldn't bank on it.
Seems he got better and better for many people by not playing a lot. Which is always a warning sign. Lets see what Oles plan is for him. He seemed adamant to not let him go on loan or something. I hope there is a plan that isn't just hoping that DVB succeeds at becoming more like Matic, Fred or Pogba. As it is, we are already very top-heavy as a team, and we have to make use of the firepower while keeping the most balance we can. And the only way I can imagine DVB being a part of a balanced midfield is when Telles or Dalot are choosen to play left/right wing in front of Shaw/AWB. A pretty high price for virtually a non-entity in DVB.