Because I'd like Nando to be a nominal part of the backline and Yaya to sit deepest in midfield bar him allowing Gundogan to break forward. Nolito and Sané can't do that, no bother with either of them playing though if they're picked, Nolito has done the business every time he's played and Sané was superb yesterday.
I dont see Yaya adding much sitting deep. If we have Fernando in front of a back 4, I think that is sufficient cover. Yaya IMO best as a late second half substitute when the other 19 players have slowed to his normal pace.