http://na.op.gg/summoner/userName=Yenn
So you can actually see the game the OP is talking about here.
Notice how he was not, in fact, six-slotted, but the Yasuo was.
Notice that he built mercury's treads into a very fed Yasuo (and Jhin) instead of ninja tabi, even though literally the only CC they would have a noticeable effect on would be Jhin's W.
Notice that he didn't build a Randuin's Omen, despite the fact that the enemy team had two crit-reliant champions (Yasuo and Jhin) and three attack speed reliant champions (Yasuo, Jhin, and Aatrox).
Notice that Yasuo literally stacked lifesteal (which would be less effective if the OP had actually built to negate it with randuin's or ninja tabi, or if he or his team could capitalize on Yasuo's relative lack of durability by bursting him in team fights, Yasuo's build is hilariously stupid but it is pretty strong in duels).
Notice how Yasuo build a spirit visage to capitalize on his hilariously high amount of lifesteal, and to mitigate a lot of Shyvana's main damage output.
Also note the blade of the ruined king, for extra tank-busting fun.
OP played badly, and got styled on by a Yasuo who took advantage of OP's poor choices. What's the big deal?