Theory - Kai'sai isn't getting nerfed yet, because Prestige Edition is coming out.
Honestly, let's look at Kha'Zix for an example - here we had a needless buff right before his 20+ Chromas released just to boost his popularity.
What's the best way to make sure the first Prestige skin sells as many Worlds Pass's as possible? Make Kai'sai stay the most played/relevant ADC up until it's release.
What other logical reason can you use for being untouched patch after patch? Any money says she won't see her first nerf until after the skins in the store.