Because you don't want to overload a champion?
Damage nerfs or buffs are easiest because it's usually the primary cause of them being too strong or too weak. If yasuo is too weak do we add more to his already overloaded kit? No, cause it's his damage, survivability or the meta that's causing it, not something missing from his kit.
For example, if we removed the input command for Karthus Q(meaning made it so he can move and auto attack with no animation for Q cast) that results in a slew of other problems. First, you can't tell Karthus is casting anything, everyone else has an animation for casting an ability, why not karthus? Second, the fact he has to stop to use his Q, even if only for a split second, makes kiting as him a bit more difficult, and it's already really easy for him to do so removing that removes an inherent weakness that was intended to be there in the first place.
The only time i understand adding new effects is when that effect strengthens their strengths, not buffs their weaknesses, such as if we wanted to buff Evelynn(she's good right now though but it's an example, calm down.),a really simple buff is to make her R have a slight cooldown refund if she kills someone, simple, her R is up again a little bit faster, it doesn't remove a weakness of her's and it rewards her for playing properly. These kind of ability changes, are good, removing weaknesses or buffing something that's already too strong, are not which is why you have to be really careful with ability changes, where as base stat buff/nerfs or ability damage scaling/flat damage nerfs and buffs are a lot easier to manage.
Edit: Like i don't think people understand how much harder it is to manage and tweak added effects over simple damage and stat changes, it's so much easier and more often than not it gets the job done.