Tielemans was getting a lot more minutes the first couple of months. 
In a weaker squad (even Zaniolo was getting starts ahead of Youri in his first few months with us), and his performances were less than inspiring until I think November time in his first year with us?  Then Emery starting tweaking the formation to include him instead of a wider midfielder, and he went from strength to strength.
Plus, he was an established international with a top-5 national side and had 150 Premier League appearances to his name.  He had plenty of experience before coming to us.  Elliott doesn't have that.
I don't think it's necessarily a bad thing that Elliott hasn't yet thrived in our side, and our set up.  He's clearly talented, and as with any talented player, I'd trust Unai to get the best out of them (eventually), even if it doesn't happen overnight.