Really? That's quite obvious.
Maybe because ter Stegen was awesome in his first season and solid in the second part of the next one? He was fucking up a lot during the first half of the second season, that's true. He also did that this season and looks like he is set to have a good second part again. Bravo was bad in the second part of the last season, remember that.
Mats was given CL and CDR before he even played a game here.
So: coaches didn't know whether Mats will be good, average or failure and they gave him 2 competitions right away.
Cillessen?
Again, the board doesn't know whether he will be good, average or failure and he was given only the least important competition.
(Even though Cillessen was already a Dutch NT coach when he came here, and Mats was a guy from an average Bundesliga team and nothing more, to some extent, except having a lot of potential on paper).
So, Cillessen's career when he came here was better than Mats' when he came here.
No matter how you look at it, Mats was given more chances from the day 1.
Also, let's say that Cillessen will play awesome this season.
Do you think that in the next season Mats will get only La liga then, and Cillessen will get both Cl+Cdr (like Mats in the past)?
I don't think so.
If you get my point?
No matter what Cillessen will do, Mats is untouchable for some reason.
Also, Mats was given 2 competitions from the day 1, and Cillessen got only 1.