Cause Oda portrayed him as rival to Luffy. Take the simplest of the simple facts we have so far: Kid has CoC, confirmed by Kaido. Only this already puts him into some elite-group. Oda is hyping this shit for a reason.
And you do a mistake here, Zoro is not one of "the two main characters". The MC is Luffy. Luffy on his own. Not Zoro, not Kid, not Sanji, not Kaido.... Its Luffy. You can argue that Oda isnt taking simply "one MC", but then it would count the whole crew. Which was kinda a fact before time skip, but since there, its absolutely clear there is only one MC and this is Luffy. Every other character is a supporting character storywise. Including Zoro and Kid.
First of all, let's pay tribute to 1000 chapters of Zoro
This "main character" thing? Don't get caught up in nonsense semantics.
Chapter 1: Romance Dawn
Chapter 2: Straw Hat Luffy
Chapter 3: Enter Zoro
Zoro is the first supporting character met, the only one Luffy sought out by name, and the one with whom he intertwined his goals. Unless you think his original offer, for Luffy to commit harakiri if he got in Zoro's way, doesn't count. Unless you think vowing to Luffy with the understanding that becoming WSS is alongside the Pirate King doesn't count. Unless you think his quote on ambition to Kuma isn't character defining. Unless you think Mihawk's comment on finding something greater than his own goal isn't character growth. Unless you think constantly warning Luffy and joking about getting a new captain if Luffy isn't a conqueror is all to be forgotten. There is technically only one main character but there's no One Piece without at least a handful at the core.
Oh and that's general stuff but not this arc right?
Oda immediately starts Wano with showing Zoro and having Luffy team up with him again upon reaching it. Zoro essentially covers all of the arc specific characters that Luffy doesn't and where they don't overlap. He's the one with an actual tie to the country, and the other one when it comes to actually preparing to fight Kaido right here. He saves Luffy from an ass kicking as soon as the damn raid starts with a statement about making it up to fight Kaido. He's right there to save him through the fight, with two emperors, while rattling off the hits that make them stand up. No boasting, so you know Oda is taking him seriously about seeing this fight through. Aside from Kaido's statements about not seeing another swordsman as strong as Oden.
Back when the shipwright debate was going a lot of people liked generic ass Pauly talking about "oh they had a moment while they were trapped together." Reminds me of people putting up Kid based on the prison thing, same shit down to the goggles.
Don't overestimate Kid Lmao. Fucking Chinjao had a name-drop as a conqueror too.
