Why is Yasuo so weak and has a low win rate in diamond+ elo?

Because people understand how to counter him there, whereas in lower elo's he just stomps freely and goes 0/15 in lane then goes and 1v5s

Because people at that level better understand the concept of counter-play, and how it's more than just a binary Champ B beats Champ A.

I think in higher elo players understand Yasou's mechanics more. They understand not to line up for a 5 man tornado, and Yasou's limitations early game before he gets his "Big" items.

This is generalized of course. A Yasou player can still be very successful in higher elo matches if he plays his card rights. A late game Yasou is a late game Yasou; a force not to be overlooked. In this current season, it seems that matches tend to be more one-sided with the "late game" being what was traditionally "mid game". As a result, I think we're seeing a lower win percentage from Yasou players because of his late game god status once he gets the right items. (i.e IE, Static, etc)

People are not retarded and feed a champion with one of the lowest base health stats of his entire class. Like, it doesnt matter if he is mobile or has a windwall if you can just shit on him with most champions because of that.