Because Fiora needed a rework and Vayne is insanely strong because HP% true damage is a problem
Riot balancing team thought it was a good idea to give her also HP% true damage which is broken by default because you cant itemize against it, perma dashes, a free banshee that on top of that mirrors the effect, and to make sure she is completely obnoxious an AoE heal.
Maybe its time to make it a goal for champ designers to design champs who are also fun to play against and not only to play yourself.