Why ?
Because Green Bull has been embarrassed. Zoro's enemies are always made to look badass and competent until their defeat by Zoro. Sanji's enemies on the other hand, while very powerful, are often played for laughs and made to looks silly before fighting him. Mr. 2, Absalom, Queen, the pattern is clear. Zoro's opponents are never embarrassed before fighting him. By contrast look at King, Mr. 1 or Ryuuma.
Kizaru has always been made to look badass, never sustaining any damage in Marineford and having several badass moments throughout the series. GB has been embarrassed by Shanks and made to look bad so he will definitely be Sanji's fight.
It also helps that he looks goofy like many of Sanji's foes unlike Kizaru who has a cool design.
Because Green Bull has been embarrassed. Zoro's enemies are always made to look badass and competent until their defeat by Zoro. Sanji's enemies on the other hand, while very powerful, are often played for laughs and made to looks silly before fighting him. Mr. 2, Absalom, Queen, the pattern is clear. Zoro's opponents are never embarrassed before fighting him. By contrast look at King, Mr. 1 or Ryuuma.
Kizaru has always been made to look badass, never sustaining any damage in Marineford and having several badass moments throughout the series. GB has been embarrassed by Shanks and made to look bad so he will definitely be Sanji's fight.
It also helps that he looks goofy like many of Sanji's foes unlike Kizaru who has a cool design.