Zoro can cut thousand Picas, that doesn't change the fact if you are loyal and number one fanboy of Luffy, then you take the second seat.
Law becomes loyal to Luffy, he took the second seat once. It's all about being loyal to Luffy.
Unfortunately, Sanji kind of betrayed Luffy, that's why he loses votes too much, but he is still more popular than Nami and others, if Oda draws Sanji as loyal as Zoro, then he would be the number two. Sanji's true loyalty is for ladies. Zoro's loyalty only to Luffy.