Because I claim that what makes Zoro a nightmare for anyone is not his AP but his complex combat style that people cant defend from.
His AP is shared by several other top tier swordsmen but his combat style is not replicated by anyone else. Difference, big one.
Also, I dont think Asura is needed for Yamato, neither does she have complex combat style nor is she hard to cut like her father.
Santoryu is more than enough to defeat her. Sure, mythical Zoan abilities should be counted in but so far, there is nothing of importance.