Yasuo was one of my favorite champions for a long time, and was the target of repeated nerfs both direct and indirect.
First, Yasuo got pummeled with direct nerfs pointed at Skill Cooldowns, Base Health, Flow Generation, Flow Shield Duration, Bonus Damage on Sweeping Blades, Flow Shield Duration again, as well as his base movement speed. These nerfs were done almost patch by patch in succession until he was extremely unfavorable for most to play, some of us still played him.
In 4.21 he saw a slight resurgence in play following a minor buff, but one that was good for his quality of life overall. Then the item change came along. in 5.1 the critical strike chance of infinity edge was reduced by 5% (Which meant an overall reduction of Yasuo's critical strike chance by 10%). This meant that you needed to find another way to reach 100% crit chance on Yasuo, or you decided to simply deal with the reduction in chance and risk not hitting a crit (80% and 90% are still pretty solid chances to critically strike after all.)
So now, your old build was 
with other items involved, paired with I believe 5% of your critical strike chance coming from runes that made you reach 100% critical strike, I'd often build 
and 
in addition to my boots, because the shield and lifesteal in combat paired with 100% critical strike chance kept you alive for quite some time and Yasuo had something like 85% armor penetration if you hit someone with his ult, you even cut through Rammus like butter.
Now his build is up in the air, do you build the same way as before and deal with the fact that you have a 10% chance to deal substantially reduced damage? Do you find a way to itemize with it, 
and waste the extra 10% critical strike chance? Throw in a 
which adds 3700 gold to your build cost? The second and third options also leave you extremely limited in item slots, what do you give up in return?
Yasuo is just not that strong right now, because it's extremely difficult to itemize him properly, and not be a glass cannon that instantaneously dies in combat. It's just unfortunately not a good time for Yasuo players.
I'll also tell you that he's far more favorable to mid lane than top lane, he has a lot of mobility sure, but only when there are enemy minions to dash through making him very vulnerable in a lengthy lane where let's face it, he'll fight against tanks or bruisers, both of which are too difficult for him to take down until he gets some items that are rather expensive and difficult for him to get to without achieving some kills to hit the power spikes in a timely fashion.
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