One Knock Up - One Kill - A Refurbished Yasuo Complaint.

I would assume that during the early game around where mid laners would be level 6 - 8, being under the turret is your safe haven till people start building tank or can pop enemies.

This wasn't the case for dear squishy mages. Instead, Yasuo can ultimate an opponent under tower and leave with a single hit of a turret that is pretty much nothing.

Why can't Riot nerf his ult where players would have to carefully decide whether they can dive in or not?

I would assume it's because if he did take damage during his ult, then he would be able to get CC'd or be flat out useless in team fights.

I do not know. But I know for one thing is that a champion who can dive UNDER the turret like that without taking any risk is horrible.