Because if he was in Wano, he'd be Law's opponent. Read what I wrote.
Law's opponent > Zoro's opponents/Sanji's opponents.
First of all, this is terrible logic. You're using 'what-if' scaling rn. Doflamingo was not on Wano, so why are we talking about a version of the manga that does not exist? If Doflamingo was on Wano, Law may still have to fight Big Mom as Kid cannot handle her by himself. Your argument for Doflamingo > King is legit based on your fanfic lmfao.
Secondly, Kaido was Zoro's opponent on Wano too. He failed and defeated King. Law fought Doflamingo; he failed and defeated Trebol. In the same arc that Zoro defeated Pica. So in the same arc, Zoro has already defeated an opponent stronger than the person Law defeated.
Thirdly, using your set-rules, Zoro usually fights and defeats the strongest subordinate. If Doflamingo was on Wano, and he was stronger than King, then Zoro would have to fight him as he'd be the strongest subordinate. Law would fight the second strongest antagonist according to your formula, and that would be Big Mom. So even your fanfic doesn't make sense. Not that Doflamingo is stronger than King; the very notion is laughable.