Mihawk had become a plot problem for the story, he was clearly created in East Blue as a goal for Zoro, and that made sense at the time, but he and the whole WSS plot just become a totally random plotline that not even Oda seems to care, as we dont meet a single character besides Zoro that even talk about it, or any other swordsman that Mihawk have defeated.
And because of all this lack of development on this WSS plotline it was really hard to fit him back into the story for the fight against Zoro without it feeling totally random and disconected from the other SH plotlines, especially as he isnt part of any of the three final organizations the SH can fight against (the WG, RHP and BBP).
Like imagine the SH are fighting the WG in the final war to save the world, while Zoro is doing a sidequest against Mihawk for his own personal sake, would feel totally out of character for him.
Sadly for Mihawk i wouldnt doubt that Oda just cut his plotline and made Zoro fight a adult S-Hawk as a way to make Zoro fight a stronger Mihawk at the same time that he can keep the order of enemies in a way that made sense.