It is about the character of course. It is also a recurring form of character you find in every shounen, I mean the cold badass but with a hearth, in some may rival/best friend of the MC. Be it Vegeta, Zoro, Sasuke, Killua etc. The point is that this type of character also need to be very close in power to the MC otherwise it becomes a joke, just imagine Vegeta and Goku relationship but Vegeta has the power level of Crilin.
While Sanji is at best the 3rd strongest guy, but who falls in the importance general hierarchy by at least a position because the main woman/ love interest of the MC comes first: Sakura before.. Rock Lee most likely? Or that pale guy in the 2nd half of the story (Sai?). Nami comes before Sanji (no need to explain to a OP community), the trio which is at the base if OP is Luffy, Zoro and Nami. Dragonball is a bit weird since Gohan and Piccolo share a bit this 3rd spot while there is no real female character who steals the 3rd place (maybe Bulma but I am not too sure). You get the point. This is also a mix of a fighter-cool character and comedic relief to not overlap too much with the MC and the 2nd guy.