Sanji has way more fans. If he didn't fight King, he would fight one between:
1) Queen: a fat and ugly guy with no neck and who is a walking gag. Don't get me wrong, he is the best new character in Wano, the most interesting one and he is as strong as King, or just a bit below him, but he doesn't fit Sanji's style at all.
2) Jack: a walking L that didn't achieve anything in 200 chapters and that is the only enemy, alongside Apoo, who was already defeated in Onigashima.
He is obviously a monster but certainly not the best opponent to hype a character without him low diffing Jack.
If popularity counts, Sanji should be the one getting King, not Marco.
If the "seemingly weaker guy defeats the stronger one after a PU/realisation" dynamic counts, Sanji should be the one getting King.
if plot counts, Sanji should be the one getting King since he pissed him off so much