Zoro is far from an underdog vs King though? Like he's currently an even bigger favourite than Marco:
- Homurasaki completely neutralises King's Wildfire
- Zoro is clearly a decently more powerful swordsman
- His Hiryuu Kaen terrified the Yonkou and is the only attack Kaido dodged (and which Mama felt the need to warn Kaido about)
- His mid tier technique (Kokujo Ou Tatsumaki) completely overwhelmed Kaido's Tatsumaki and badly cut him up with Kaido coughing up blood for 3 pages
- He's still adapting to Enma so he'll become even stronger
- He still has Asura
After the last 3 chapters, most people who read Zoro vs King would go in assured of Zoro's quick victory. Like it's not even clear that King gives Santoryu Zoro a solid high difficulty fight, let alone after Zoro fully adapts to Enma or if he gains the Nidai Kitetsu.
Marco vs King is more up in the air than Zoro vs King. Giving Zoro the ability to cut fire already removed most of the tension for the fight. Giving him offensive prowess that can terrify the Yonkou solidified that he's on another level entirely. Zoro vs King is the furthest thing from an underdog fight.
How exactly do you expect King to challenge Zoro?
I think the true difference is that while yes, Zoro has cut Kaido and I also understand the implications of that, its more of giving Zoro the "right" to be stronger as part of a good growth metric.
What I mean by that is, we saw Luffy progressively get stronger, fighting Shichibukai of different strengths, to a Yonko commander, now to a full fledged Yonko. At some point Luffy is warranted some form of a 1v1. Its bound to happen and has yet to ever been taken away from him. Even Thriller bark, which had the whole crew fight Oars and Moria, still ended with a a fight between Luffy and Moria.
I just want the same logic applied to Zoro. I know Zoro is much stronger than he's let on so far in the new world. That much is entirely clear, but its about gate keeping. I think its a good "metric" to at least give Zoro a 1v1 fight against another swordsman, a Yonko Commander at that. Its been 550 chapters since his last decent swordsman fight (Ryuuma). I just personally want to see him surpass King, a meito user. Even if its a disadvantage (Zoro losing a sword or heavily injured from Kaido, etc.), I still think its a good idea to have.
Not only this, if the pattern is to have the other Strawhats (Yamato included lets say) fight the Beasts Pirate executives exclusively, then to me it just makes sense for a Strawhat to fight King. And if thats not Sanji because he's fighting Queen or Jinbe because he's fighting Jack/WsW, then it really should just be Zoro. I personally can't see Marco taking over the fight permanently. Its not Oda's style to have 2 similar characters in terms of strength just up and fight each other and dedicate time to it. It really doesn't get us anywhere because Marco is a secondary character in the story with no plot motivation to take out King (as opposed to something like Kyros vs Diamante, which was entirely warranted)
Its more of thinking about it in a rudimentary way: The Strawhats are the main characters and thus far, deserve every single fight against the Beast Pirates as a result. Zoro vs. King is 100% not a knock on Zoro. In fact, its the opposite to me. I'd personally love a solo battle to finally happen again for him. I'd also love for him to get that "moment" that everybody wants for him against Kaido. But again, entirely dependent on how Oda does the roof fight. I just think its way to early to determine whats going to go down up there.
Tldr: I just want Zoro to finally have another good swordsman fight and "grow", even if its at a disadvantage or something.
Edit: Also really, we don't really know much about King or how strong he is lol. Oda has written the commanders in such a way that all 3 have barely been able to show what they are made of yet. I don't think Zoro cutting Kaido particularly means he can't struggle against King or have a competent swordsman fight. Kaido has taken a stupid amount of hits from Luffy so far, yet someone like Doflamingo or Katakuri have dodge far more hits and taken less damage overall with other skills on display. Its not l'm trying to say King then has to be a god to fight Zoro. It can just be a more competent fight that doesn't just involve Zoro wailing on Kaido and us finding out it doesn't work (which so far, it hasn't been enough yet). That is boring in my opinion.