Zoro shares parallels with Luffy while Sanji doesn't: Zoro and Luffy are both Kings, both WG Supernovas, both Conquerors, both Advanced Haki users, both have top tiers as their precise benchmarks to reach and surpass, both have fighting oriented dreams. Sanji completely lacks all of that, Oda didn't want to give him but he gave to Zoro.
Zoro is showed, stated and implied by multiple sources (manga statements, datsbook statements, the 1031 cover with the Number Twos) to be the second strongest in the Strawhats, Luffy's right hand man and the first mate/vice captain for the rest of the crew.
Zoro always performs better than Sanji against strongest opponents or similar opponents: just compare Zoro vs Fujitora and Zoro vs Kaido and Big Mom to Sanji vs Doflamingo, or Zoro vs Pica to Sanji vs Vergo. Why, you think?
Zoro is also the one who, as the first mate/right hand/second strongest fights to protect Luffy from the very strongest enemies, especially when Luffy is defenseless (Zoro vs Kuma and Zoro vs Kaido are prime examples of this).
Zoro is the one who shares with Luffy some of the most iconic scenes, while Sanji is not present: the revelation of Blackbeard with the "is not he, is they", chapter 1000 for the great fight between Supernovas and Yonkos, he is the only one Strawhat who met both Ace and Sabo.
Databook states that Zoro has strength on Luffy's level. Another databook ranks Strawhats in fighting strength and gives Luffy and Zoro 6/6 while it gives Sanji 5/6.
Zoro most of times fight someone that Sanji can't defeat (in Alabasta Sanji would brutally lose to Mr 1 who Zoro defeated, in Skypiea Sanji would lose to Ohm who Zoro defeated, in Enies Lobby Sanji would either lose or at least have an extreme diff fight with Kaku who Zoro defeated, in Dressrosa Sanji could not defeat Pica who Zoro defeated, in Wano/Onigashima Sanji would lose to King who Zoro defeated).
Sanji will never be as strong as Mihawk, let alone stronger than Mihawk, when Zoro will 100% be stronger than Mihawk.
Sanji never fought someone that Zoro couldn't defeat (on the opposite, Zoro would make a clown of Sanji's opponents; while, as said, Sanji can't defeat most of the enemies that Zoro defeated in the same arc).
Also Zoro always has a fight in every arc. Sanji does not. And Zoro has many more power ups than Sanji, and greater power ups also. Why, you think?
Zoro: in Logue Town receives stronger swords than the ones he had before, in Alabasta learns how to cut steel (CoA Haki), in Skypiea learns how to do flying slashes, in Enies Lobby learns Ashura, in Thriller Bark receives Shunsui which is a much stronger sword than the one he had before, then in time skip trains under Mihawk, and in Wano/Onigashima receives Enma and unlocks Advanced Conqueror Haki.
Sanji: in Enies Lobby learns Diable Jambe, then in time skip trains under Emporio Ivankov, in WCI receives the Raid Suit and in Wano/Onigashima unlocks the esoskeleton and learns Ifrit Jambe.
Zoro's main iconic power ups are greater than Sanji's main iconic power ups (Ashura >> Diable Jambe and Advanced Conqueror Haki >> Vinsmoke Esoskeleton and Ifrit Jambe). Not to mention Oda having Zoro training with Mihawk... while at the same time Sanji is training with Emporio Ivankov.
Zoro has much more parallels and things that he shares (portrayals, powers, goals, benchmarks) with Luffy than he has with Sanji. And most things that Zoro shares with Luffy, Sanji has not. Why, do you think? Maybe the answer is just the easiest...
Not to mention that Sanji was sure that Pre Adv CoC Zoro could have killed him if he went crazy with his newfound powers and started attacking people without realizing who they were.
Meaning that Sanji himself admitted that Zoro is confortably stronger than he is. And that was Pre Power Up Zoro, let alone the Current one.
B-but they bicker in gags! T-that makes them rivals and equals!