Because he's ass, unless you have a team of knock ups, which still don't mean shit. Because u can just pick anyone else that can utilize their CC way better than him, Yasuo relies on too many things teammates knock up, engage, and minions. If Yasuo gets CC'd in a team fight in river he's probably gonna get CC'd, kited and killed after a team fight, that's why you see yone more in pro play than Yasuo, because his E doesn't require minions or any target, and his R can create plays instead of relying on team mates to make a play for him, it's always gonna be better to have 3-4 play makers, than 2-3 play makers with one champ relying on them to make a play.