Because it's just too out of place for her to be the only one that can be turned. She was not origin for a long time, which is why I think she was planned to be "dedicated mindflayer" because the fix was never planned and cuts had nothing to do with that. For example, Astarion's content was moved from upper to lower city, why couldn't they do the same for Karlach? She barely has any content in two previous acts, if something that would affect her story was planned for upper city, it wouldn't be much and moving it would be far easier than Astarion's. That's why I think if anything is to be changed, they'll have to rewrite her. Because they didn't plan her to fixable, now the question is whether or not they'll listen
I don't think she was planned as "dedicated mindflayer". Last I checked, you have more options than her. Last I checked, you also weren't forced to bring specific characters to the final battle, which makes that whole option just that: an option. You can not simply take one option, that you are not even forced to take, and that is entirely depending on your own choices and your party composition, and then draw the conclusion that that was the whole purpose of said character. That is not how it works. You can take so many different paths with your companions throughout the game, make so many important decisions about the direction you want to take their story, but somehow this one tiny little alternative being presented to you makes you all hellbent on this being the true purpose of a character and the only possible and intented conclusion to her whole arc.
As for the whole rest, Norrec already explained the flaws in that argument.
