Why do professinal players play their worst champs in tournament?!

Slooshi is great with Ziggs and Kassadin, so why does he keep picking (And losing with) Azir? Three times against Cloud9 he went in, Emperor's Divide, died. If they could have a casual and candid conversation with Cloud9, I'm sure they'd be more than willing to tell him, his Azir is so bad, that focusing and killing him whenever he appears was effortless, which anyone could see when watching this.

All his coach needs to say is "Don't play Azir, ever again." And their team can start winning. This is as bad as when Lemonation kept playing Morgana. It just doesn't work! If you even win your team clearly carried you.

EDIT: OMG OMG they played the feed speak of Cloud9 And the guys literally say "Kill Azir, kill Azir" holy crap how is T8 or their coach not getting this? Don't play Azir anymore!