hmm
In terms of personality, Zoro looks to us more to be a leader than Sanji!
Yet the reality of the matter is that Sanji lead more times than Zoro did during the story. Wano is a great example where Zoro SHOULD have showcased leadership when Luffy was absent, but he simply was a lone wolf!
So, for us, Zoro's seriousness and strong will...etc! they all strike us that he's fit to be a leader more, but when you look at what is happening in the story and "not what you think"
Zoro looks more of a lone wolf
Sanji more of a team guy
Which is why Sanji was shown leading more times than Zoro
In fact, the only time the name of the crew SHs changed into "curlyhats" was with Sanji. While during Wano because Zoro didn't lead anything, he's a lone wolf, there was no change in name like "marimo-hats" lol
But generally, Zoro fits more as the sound of reason/advisor for captain. Because he is less emotional and not allowing the captain to falter (Usopp scene in water 7)
However, being advisor is not the same as taking decisions.
So, even though Zoro may come up with a sound reason and great advise, and back up the captain the most, BUT that's a different role than being the guy with final say/decision.
There is a reason why advisors stay advisors and not become leader themselves. Because when it comes to taking decisions, even people with sound reason can falter and start losing it because taking the final say is not the same as saying your opinion