Kalista is a new answer to an old question
That question is:
Why doesn't Riot base MMR on how well you did in a game, like K/D/A?
edit: Okay, clearly I was far too subtle. Many people who erroneously think they are better than their match-made teams present the idea of basing rank off KDA. The old answer, as outlined below, is that KDA isn't a good measure, but certain people refuse to believe that answer.
Kalista now forces those individuals to recognize that KDA isn't a fair measure, because Kalista's bad decisions can directly impair the KDA of her soulbound ally. It wouldn't be fair to implement a system that punishes players for having bad Kalista's on their team. Thus, the answer to the question "why won't Riot base MMR off KDA" is "Kalista."