mainly because it only fits very awkwardly into the story.
when would Zoro meet Mihawk again?
why would he abandon his crew to fight an insignificant battle?
is there even any tension left?
3 possibilities from my PoV.
1. Shanks becomes an antagonist and Mihawk joins him
2. Mihawk dies and Zoro faces Mihawks killer
3. the fight will happen after Luffy became PK
1. has the problem that it would be super weird if Mihawk just trained Zoro to end up getting offed by him in a real fight with more at stake then just the WSS title.