The reason is because of what Zoro himself states, and that is the fact that he put "everything he had" into that last attack. Haki blooms under extreme circumstances, and Zoro was near death, and his Captain was about to be killed. So it stands to reason that in that do or die moment, Zoro unlocked and applied Conqueror's Haki to Asura.
Zoro should in no way shape or form not have some kind of understanding of Asura when he has already utilized the attack not once, but twice preskip, and then trained with Mihawk for two years. What he did against Kaido surprised even him, and Asura should be no surprise to him, especially since he even names his attacks "9 Sword Style", so it's not like it can even be argued that he was not consciously aware of it when he used it against Kaku and the Pacifista.
Another thing, is Zoro is all about getting stronger. When he unlocked Breath of All Things during Alabasta, we specifically got a scene at the end of the arc of him acknowledging it, and proclaiming that he needs to be able to unlock that power at will in order to become stronger. So for Zoro to experience something like Asura twice, but yet do absolutely nothing to try and tap into that power and use it at will to become stronger is extremely out of character for him, especially considering that he spent two years training with the World's Strongest Swordsman. If there is anyone who could help him get a better understanding of what Asura was, it would be that guy, so we could rest assure that he would have brought it up. It's just way to far fetched to believe that that Asura is AdCoC that Zoro had no clue even existed.