Because Zoro is making a name for himself. Contrary to what some people think, Zoro walks beside Luffy not below. He will be known as the strongest swordsman in the first instance, not Luffy's right hand man.
Luffy is currently becoming a legend, if Zoro doesn't do it this time he will never do, he will always be the legend's first mate, as Rayleigh is.
When we see Roger recruiting Rayleigh he doesn't seem like someone ambitious, unlike Zoro who long before Luffy was already dedicated to his ambition.
Zoro and Luffy will build their names side by side and not one being pulled up by the other.
Luffy is currently becoming a legend, if Zoro doesn't do it this time he will never do, he will always be the legend's first mate, as Rayleigh is.
When we see Roger recruiting Rayleigh he doesn't seem like someone ambitious, unlike Zoro who long before Luffy was already dedicated to his ambition.
Zoro and Luffy will build their names side by side and not one being pulled up by the other.