Because Zoro himself said he can’t accept Luffy as captain when he is weaker then him.
Literally everyone in the world has ryuo because ryuo = haki...
![](https://mangabeast.com/manga/One-Piece/0597-012.png)
We have the red scarbads who are masters of flow ( CoA Haki) the top 3 have the version of Ryuo that Luffy was learning before the version of Ryuo that allows you hurt the enemy from the inside.
Hyo the "mighty blade" the one who teached Luffy what he currently knows.....
A country feared by the strength of the Samurais...
But not a single one of these could wield or tame Emma ...
Only the past Shogun who scarred Kaido managed to do such ...who obviously was a CoA master ( even advance I would say).
Unless you think Oden tamed Enma with basic hardening....
Zoro has advance Haki...only one with Ryou above the rest could have tamed Enma