I don't think Zoro has ever shown interest in "fighting" the main villians before. Morgan, Arlong, Enel, Moria, Doffy. They just happened to cross path with Zoro, who of course would fight them in order to achieve his other objectives (stalling for Lufy, saving Robin, saving Law...). But now his MAIN objective is to fight, defeat Kaidou even. That has never happened.