What's the part that doesn't make sense?
Why are you pretending like we don't have someone who can fight King when Marco is right next to King.
Zoro will go fight Kaido alongside Luffy the way it's been foreshadowed for 90 chapters.
Zoro not getting his own fight where hes able to shine the most would be a waste especially if every other strawhat but him gets one. Hed definitely hurt kaido but it it shouldn't be his main fight of the arc
Against kaido not only is he getting overshadowed by luffy he'd have to share what spotlight he has with luffy, kid, law, killer, and yamato
Again if sanji fights queen, jinbe fights jack, and the rest against tobiroppo its not gonna be Marco fighting king its gonna be zoro
Plus marco fighting king would be a waste of both characters