Because we have all had those 1/15 Yasuos who are toxic fuckwads. Yasuo, Riven, Lee Sin, and Vayne (And to a lesser extent Yi, although he tends to be less of an issue just due to low pick rate unless OP) all attract players who tend towards high toxicity in games because they are all champions that have potential to virtually solo-carry a game. Combo that with the fact that when they are good they can get to 40%+ pick rates rather easily (Not counting bans), it's easy to see why people hate them. They always either feed so hard that they lose the game for you hands down, or they end up 10/0 and you may as well not be playing because you are sure not doing anything that game that is going to have an impact compared to them. Or worst of all, you end up with one with a score like 16/15/0 where they go "I'm not feeding, I have a positive KDA!" who has effectively inflated the gold on the entire enemy team and throws the lead they had built while they complain about their team doing no damage because they are all an entire item behind the enemy team almost. And before you say a score like 16/15 isn't bad, it means they have given approx 5000-6000 gold to the enemy team, while having obtained something in the range of 3500-4500 personal gold from the kills.