In case you skip the training arc, Zoro already beat every other island inhabitants, getting cared by a beauty, and a WSS ready to train him. And people often missed this, he has access to (alcoholic) beverages, something he loves, during his training.
Sanji on the other hand, still need to beat 99 masters, getting chased by offenders, no one to train him, and doesn't have an access to women, something he loves.