But recent champions having a lot of personality/being too memorable.
Let's face it, all these champions have VERY distinct personalities in LoL. Kai'sa don't have a personality as strong and as memorable as them.
The problem is that we have no idea how Kai'Sai survived in the most horrifying realm of runeterra's universe, the Void itself without going insane.
This was a big thing around when Taliyah came out as well.
Kai'sa being less bombastic is something I think they can afford, now that longer lores are coming out and we have opportunities to flesh out characters outside of VO.
I feel like she's just forgettable, which is the last thing I expect out of any League champion. Hell not even Skarner (before he was busted) or Volibear are as forgettable as her.
The main problem is her voice actress, who did a poor job (or, at least, it didn't meet the standards we're used to).
You forgot 
Personality doesnt really indicate the popularity of a champion, Orrn has been forgotten completely, its mainly how they look like and Kai'Sa has a pretty design that appeals to the majority of the costumers
