I watched Serbia's group games and their first knock-out match in my city for the U20 World Cup. My suspicion is that Serbia went for specialist midfielders - Zdelar, the captain as a no. 6, with Maksimovic ahead of him as a no. 8 and then Milinkovic, the most talented of the three playing at no. 10. I wonder if Grujic is a bit more box-to-box, and was considered to not be as good as the specialist roles? Or it may be that he's developed later than the others - he could even be a year or two younger for all I know. Or perhaps the coach had used the other three in qualifying and didn't want to change a wining team. Grujic's only start was when Milinkovic was suspended.
Another possibility is that Grujic has improved a lot in the last few months, and we've woken up to that ahead of other teams. We need a Serbian specialist to answer this properly!