I think that he simply never fought her for real. All there was was some scraps here and there (including the Wano one), but they never really went to hurt each other. In the end, he probably considered her like a big sister (and obviously thought that he would certainly beat her in an 1v1).
In short, the main reason is that he never seriously faced Big Mom. It's the same reason that Garp isn't there, despite being responsible for beating one of his "top 5". He can only measure who he faced.
In short, the main reason is that he never seriously faced Big Mom. It's the same reason that Garp isn't there, despite being responsible for beating one of his "top 5". He can only measure who he faced.