Blah, blah, blah; the hard facts here are:
—No author focused on fighting scenes has cared to draw an original spin-off for Zoro; the closest thing was that short remake of his first fight with Mihawk. Meanwhile, Sanji is interesting enough for another author to write almost 300 pages so far of brand new content for him.
—Oda writes a 150+ long arc focused on swordsmen from feudal Japan and even hints at Zoro having more backstory and links to the Shimotsuki, yet decides to sideline him (no gathering the samurai, no visiting Ryuma's grave, no avenging Yasu, no becoming the new Ryuma...) and openly discards any intention of exploring his character further in a SBS. Like he doesn't even have the desire to work with this character in a deeper way.
—He also takes the trouble to develop a character arc for Sanji through Zou, Whole Cake and Wano, to the point of focusing his fight against Queen on closing his conflict about using science and becoming what Judge wanted for him. Meanwhile, Zoro just does more of the same: fight, fight, fight. For an author who loves characters over battles, Zoro's battle feats (which are ultimately pointless as he still ended up having the expected Bonez/Kaku as his actual milestone for this arc) pale in comparison to Sanji's character moments. I mean, no matter how much your gang laughed at it: I'm convinced Oda would choose the "Robin, help me!" moment over any "King of Hell" feat Zoro had.
(Note: I'm not saying Zoro needed more backstory and such; personally I didn't even like what he did with Germa and Sanji, but this is about Oda's intentions, and so far he chose to give Sanji a whole additional arc, a whole additional backstory, a whole additional character arc... while he put next to zero effort on Zoro. That should tell you something about which of the two he'd rather write about).
Again, I know you guys love to bash Sanji. But when Oda has the chance to develop his characters, to build psychological arcs for them, to explore his themes and concepts..., in other words, to actually work as a writer, it's Sanji who receives all the attention. He could have done tons of things with Zoro, every theme this character embodies was present in Wano (even those touched upon during his flashback with Kuina); yet he chose to do nothing but, in the best case, hint at stuff to later discard most plot seeds he planted for him.
Should tell you something.