Or..... Kalista..... like Nidalee, Elise, Irelia..... her tasking gives people headaches in micro management, and very real hand conditions caused by typing in a specific pattern or trying to master control over their positioning.
Senna is new. Kalista never ever even had a player base, even when she was broken. Something about her design never appealed to the broad player base. Kind of like how Zoe doesn't appeal to the general player base. Or how even though potentially strong, Zilean never has had player base appeal. How often do you see a Skarner?
The fact is, aesthetic appeal is always a factor, and Kalista doesn't have it. She's monochrome, her voiceovers are bland, her skill set isn't flashy.... and she takes a lot of work to play.
I'm going to sound biased for voicing my opinion, but League only has a few appealing girls who pull off the pretty nonwhite nonasian theme:
Pulls off the big strong athletic girl.
pulls off "thicc" and more culturally current.
pulls off the perfect 10. Kalista has never had any of those appeals (and is probably why Senna was reinvented with a Fleshy character!). And notice..... all three came after Kalista.