Throughout the series, it's been described that people who possess the "qualities of a king" are those with unwavering will, those who can impose their will on others, those with "kingly" aspirations/ambitions, etc. I don't think it's an exaggeration to say Zoro and Luffy are the legitimate embodiments of these traits. Zoro's also had a plethora of moments throughout the series where his conqueror's haki was hinted at
With Sanji, he doesn't share these traits. At the very least, those traits aren't fundamental to his character like they are with Luffy & Zoro.
