#2 Good competition for the center back spot
Strange, but still true. Barcelona has three really good center backs at their disposal. Gerard Pique is there, Samuel Umtiti was one of the best in his positions till the injury came, Clement Lenglet has more or less stolen Umtiti's spot in the side. When de Ligt comes into the mix, there is certainly a doubt over his position. Can he be a starter with such quality around? It depends on what the coach thinks and what he does in the training.
On the contrary, healthy competition will keep him out of the side for a few matchdays but he will eventually get his spot, at least in Copa del Rey matches. Thus, he has to prove that he is better than those around him. That is what separates the good from the greats. The good is good where there is no competition around. The greats are great even with competition surrounding them.
Think about this. Clement Lenglet was an outsider when he came into the side. When he got his chance, he integrated himself into the team and has now improved himself to a level where he is irreplaceable on current form. Even Samuel Umtiti couldn't replace him undisputedly. Lenglet has now earned his place in the side and he has earned his name as one of the world's best this season. If he hadn't fought for his place, who would know about him? If he was still at Sevilla, due to the fact that he will get regular playing time, no one would bat an eye.
If de Ligt goes to a club like Manchester United, he becomes the undisputed starter. Keep in mind, he is still only 19 and thus the pressure of the Premier League can take its toll on him and his talent. If he underperforms, the media will burn him down. At Barcelona, he will get his chance only when he is that good, only when the staff feels that he can handle the high pressing of teams like Valencia and Villarreal.