What did he achieve that others didn't? Scarred kaido's chest and fainted after wards? Kaido said with the Power of ALL Alliance by his side, he could rule the world.
Shanks stopped kaido from going to marineford, big mom fought for 3 days vs him, whitebeard was most DEFINITELY the vice CAPTAIN inside rocks' ship which already tells the what you need to know and oden only lost due to tricks.
Zoro never did anything close to this feats, he used ALL he had to get a "good job kid" from kaido and faint afterwards on the ground while kaido literally didn even feel shit, proceeded to win vs luffy, win vs yamato, kill luffy, fight gear 5 luffy while lifting the whole place and only after Luffy's strongest attack, he went down.
Zoro is WAY below yonkos and admirals aswell, he's a yonko commander, that's good enough.