Because Zoro was the damsel in distress in that moment. So that was Oda doing his thing for the sake of tension lmao. You're reading too much into it. The situation was basically kizaru saying "Ouuuu easy target".
He wasnt hyping up Zoro's strength or endurance period.
You still don't get it.
Zoro took a hit from the pacifista, in the previous chapter and fell. Yes, he did get back up and continue fighting. Now, Luffy was surprised that Zoro fell and was curious if he took a major hit. Sanji told him to drop it because they haven't defeated their opponent and he didn't know Zoro still hasn't recovered from taking all of Luffy's damage and fatigue.
After they defeat the Pacifista amd Oda shows us that all of the straw hats are tired from the fight. They had to sit down and rest.
Zoro then encounters an Admiral, who's clearly stronger than a pacifista, who drops him in a single shot. Zoro is down for the count.
Based on everything I said, which can be found in chapter 510 and 511, there's no reason for Oda to have Kizaru tell us Zoro was knocked out from one attack because he's very tired. That can be easily determined just from reading the past chapter. That means there is another reason for that and is he wanted Kizaru to acknowledge Zoro's strength.