Because of plot and Status Quo is God. If Zoro didn't wait until he's almost dead to use the form against his opponent that arc, but instead use it right at the start of the fight, then he'd be sitting on his ass for the rest of the arc because Zoro doesn't do much outside of beating strong guys up.
He used it when he needed to, and due to context, it would be enough.
Against Kaku, he used it when everything wasn't being effective enough, and it ended up ending the fight immediately.
Against the Pacifista, it was the second to last attack before it went down, so there is no need to use it again.
Against Kaido, Zoro was already at his physical limit due to Hakkai and only had room for 1 more attack.
Pre-Wano and post-TS, he was never pushed to using it. So there was no need.
Also, about the limit stuff.
The only sword style that has a limit is "King of Hell: 3 sword/dragon style", and the limit is for Enma, not Conqueror's coating.
9 sword style doesn't have a limit, it's a sword style that he can use like the other ones.
Had Kaku not went down after the first Asura attack, Zoro would have used another one.
Will we ever see an opponent take multiple Asura attacks from Zoro? Probably not.
The way most Zoro fights go is him simply beating the opponent's gimmick and/or 1-shotting them after using a PU.
If Zoro's fight was long even AFTER he did that, then it would reach Luffy-levels of durations for panel time (for a fight).
I doubt that's Oda's goal; he wants Luffy to be the focus at all times during the climax, so his fight should always be the longest, even if it's at the cost of how enjoyable other fights can be.