It's not difficult to grasp, Oda will not allow Zoro to fight and defeat an Admiral before Luffy.
Within the crew Luffy was the first to defeat a Commander, and Yonkou. Now it's time for the Admirals.
Within the crew Luffy was the first to defeat a Commander, and Yonkou. Now it's time for the Admirals.
Kizaru isn't even a villain, it's just a shallow fight with a dude who's following orders, based on his sheer strength. Akainu is the only relevant admiral and even Kuzan should be second considering his ties to Luffy, Robin, Akainu, and Garp.