Because Yasuo is poorly designed. The wind wall skill makes sense on a support like Braum, and also it has the balancing factor of only reducing damage to the first target hit (Braum) and applying CC. Braum came out not long after Yasuo, so one of two things happened during development of these 2 champs.
Possibility 1: Wind Wall was originally designed for Yasuo. Riot spent a lot of time coding the skill, and so wanted to shove it on another champion in order to justify the effort. Someone realised it made more sense as a support skill, and Braum was made.
Possibility 2: Wind Wall was originally designed for Braum, with that being his defining mechanic (taking bullets for allies). CertainlyT then was desperate to thing of even more things he could stick onto Yasuo's kit, and stole the Wind Wall idea.
I would argue that the wind wall mechanic is in itself in direct opposition to the foundations of the game (as it hard counters skill shots, which was one of the game's biggest selling points originally), but in Braum's case it feels like it makes sense, even if it is boring to play against.