Because all this arc, at the very least, as been Luffy getting ever more ready to fight Kaido. Add to that Momo and Yamato who have far more beef with the man than Zoro, who is just the descendent of some illegal emigrants from Wano from before Kaido's takeover.
Furthermore, ever since Enies Lobby, Oda has made it quite clear that Zoro is not on the same level as Luffy. Zoro's opponents have never been of a caliber similar to Luffy since Whiskey Peaks, basically, where that opponent was Luffy, in fact.
Yes, his attack, his BEST attack, the one he used to finish his strongest opponent pre TS (Kaku) hurt Kaido. But it wasn't even on the caliber of Oden's, who would have been able to finish a younger -and thus weaker - Kaido back then, without the old hag interfering.
Yes, he blocked for a few seconds the combined attack of two Yonko, which is an extremely high level feat.
But storywise, we are not following Zoro's adventures. We are watching Luffy's. He's the one who has to prove that he has what is needed to become the Pirate King and basically change the world. It's always been Luffy's journey. He has gathered an incredible crew over the years, and each and every one of the SH Pirates, unleashed in any of the outer Blues (and perhaps even Paradise) would be an absolute monster.
But they aren't the focus of One Piece, they have never been, and never will be. Therefore, it wouldn't make sense for Oda to have Luffy being the one training like a madman, unlocking incredible power-up mid-fight and always refusing to give up, even when faced by Kaido's immusearable strength, to actually be robbed of a victory because somehow Zoro would suddenly be able to one-shot Kaido.
We aren't talking about fodders here. We aren't talking about an East Blue Captain even. At the level the SH are now, especially the Monster Trio, they aren't facing opponents they can just one shot. Each and every fight is gruelling because at best they are slightly stronger than their opponents. And sometimes they seem to be hopeless, like Nami and Usopp vs Page One and Ulti !
Sending all of it away just to have Zoro kill Kaido, a Kaido who would then need to be displayed has having defeated Luffy once more, and for good, and still be in a good enough condition that Zoro killing him like Ryuma killed that dragon would be an horrible story. It'd rob Luffy of any progression or interest since all his efforts would have been in vain, it'll lead to an infinite amount of plot holes (if Zoro could kill Kaido from the get go, why didn't he just rampage all over Wano? Etc.).
Also, keep in mind that Oda already went out of his way to show us that even the WSS don't get to try to take one the strongest Emperor alive in Marineford. Mihawk, who is probably still stronger than Zoro currently is, never ever got a chance to even scratch Whitebeard.
In One Piece, an ironclad rule (as far as I remember of course) is that a "Captain" is always defeated by a "Captain" (Baggy, Crow, Arlong, Crocodile, Enel, Foxy, Lucci, Moriah, Hody, Caesar) or an alliance of captains (Doflamingo). Even Newgate was killed by Teach, with his crew's help, but he was already a dead man walking by that point.
I know my point is muddied because English isn't my first language and translating my thought in it is kind of difficult sometimes, but I hope it comes accross well enough.
Furthermore, ever since Enies Lobby, Oda has made it quite clear that Zoro is not on the same level as Luffy. Zoro's opponents have never been of a caliber similar to Luffy since Whiskey Peaks, basically, where that opponent was Luffy, in fact.
Yes, his attack, his BEST attack, the one he used to finish his strongest opponent pre TS (Kaku) hurt Kaido. But it wasn't even on the caliber of Oden's, who would have been able to finish a younger -and thus weaker - Kaido back then, without the old hag interfering.
Yes, he blocked for a few seconds the combined attack of two Yonko, which is an extremely high level feat.
But storywise, we are not following Zoro's adventures. We are watching Luffy's. He's the one who has to prove that he has what is needed to become the Pirate King and basically change the world. It's always been Luffy's journey. He has gathered an incredible crew over the years, and each and every one of the SH Pirates, unleashed in any of the outer Blues (and perhaps even Paradise) would be an absolute monster.
But they aren't the focus of One Piece, they have never been, and never will be. Therefore, it wouldn't make sense for Oda to have Luffy being the one training like a madman, unlocking incredible power-up mid-fight and always refusing to give up, even when faced by Kaido's immusearable strength, to actually be robbed of a victory because somehow Zoro would suddenly be able to one-shot Kaido.
We aren't talking about fodders here. We aren't talking about an East Blue Captain even. At the level the SH are now, especially the Monster Trio, they aren't facing opponents they can just one shot. Each and every fight is gruelling because at best they are slightly stronger than their opponents. And sometimes they seem to be hopeless, like Nami and Usopp vs Page One and Ulti !
Sending all of it away just to have Zoro kill Kaido, a Kaido who would then need to be displayed has having defeated Luffy once more, and for good, and still be in a good enough condition that Zoro killing him like Ryuma killed that dragon would be an horrible story. It'd rob Luffy of any progression or interest since all his efforts would have been in vain, it'll lead to an infinite amount of plot holes (if Zoro could kill Kaido from the get go, why didn't he just rampage all over Wano? Etc.).
Also, keep in mind that Oda already went out of his way to show us that even the WSS don't get to try to take one the strongest Emperor alive in Marineford. Mihawk, who is probably still stronger than Zoro currently is, never ever got a chance to even scratch Whitebeard.
In One Piece, an ironclad rule (as far as I remember of course) is that a "Captain" is always defeated by a "Captain" (Baggy, Crow, Arlong, Crocodile, Enel, Foxy, Lucci, Moriah, Hody, Caesar) or an alliance of captains (Doflamingo). Even Newgate was killed by Teach, with his crew's help, but he was already a dead man walking by that point.
I know my point is muddied because English isn't my first language and translating my thought in it is kind of difficult sometimes, but I hope it comes accross well enough.
