Yasuo was VERY poorly designed overall. He snowballs REALLY hard if he has a good early game, but he ALSO scales really well so he can come back very well if he has a poor one. He's never useless due to the powerful utility of his Windwall and the extra CC of his ult. He has a metric ton of free stats in his kit (free health from shield, free crit from passive, free armor pen on ult), making him play as if he has a lot more gold invested in his build than he does. He's manaless, so he doesn't need to manage a resource, and almost all of his cooldowns are non-existent so he doesn't need to worry about blowing one at a poor time and leaving himself vulnerable. Yes, GOOD yasuos are very rare. But lousy Yasuo players can still do VERY well just by playing him lousy. He is a simple champ to use and he doesn't punish the person playing him for being stupid. At low Elo, Yasuo is hard to deal with because his opponent don't know how to handle him or to pick champions that can punish him. In high Elo, Yas is hard to deal with because actual good Yas players are god-damned terrifying and can outplay even their most brutal counters.
Add in that a lot of Yas players are massive cocky jerkoffs, and you get a problem. Almost every Yas player firmly believes three things.
- That he is the most important player on his team.
- That his teammates are required to pick champions that can set up Yasuo with knock ups.
- That he is the most difficult champion in the game and thus, the best player in the game for picking Yasuo.
This leads to massive arrogance and self-entitlement from a lot of Yas players. They expect to be given kills. They expect other players to pick champions to set them up instead of the champions that those players wanted to play. They assume that everyone else is a worse player than them, because of their champion's high skill cap. They always believe they're the carry... even if they're massively behind. And the way Yasuo is designed encourages almost ALL of this toxic, selfish thinking.
etc. here) is on the enemy team, they will be an absolute god, but if he's on your team, they will feed, flame, and most likely afk.
players pale in comparison to a toxic 
