It all comes down to which fandom can spam the most
Let's go spammers
That, and how loyal fandoms are.
Zoro has wider appeal than Sanji and there are more fans that consider themselves Zoro fans than there are that consider themselves Sanji fans.
That said, I think Sanji has very loyal fans, perhaps only second to Luffy, and Zoro's wider appeal has more competition, meaning he's more likely to shed votes to other characters of similar appeal that Zoro fans are also heavily invested in.
I think Luffy and Sanji fans are among the most likely to exclusively vote for them.