User blog comment:Xilinoc/Ch. 684 - The Blade/@comment-24769684-20160728121813/@comment-1970092-20160728155745

Imagine this: Yhwach met Aizen in the first invasion. Considering Aizen apparently altered his perception of time, he probably had his Zanpakuto powers by then. And we already know that once you've seen KS once, you are in the club for life. So maybe that was Aizen's way of inducing KS on Yhwach, thus making him susceptible for all future encounters, even with the Almighty. I bet that if Yhwach had the Almighty when he first met Aizen, KS wouldn't have worked.