It's simple, he couldn't show Zoro's CoC before Wano.
Where would have been the point of it?
It was obvious that Zoro is getting a major plot as well as major fight in Wano. The Kaido fight has the most significance in all of post ts.
It was built up since forever and Kaido's fall marks both the beginning of the end of the vs Yonko saga as well as Wano's liberation.
It was not possible to show Zoro go all out before this main and narratively important fight as it would've taken away the tension and surprise for Asura/CoC.
Had Oda shown Zoro's CoC before, he would've had to come up with again another huge reveal in the Kaido fight.
Also it's true that it has been a long time since Zoro got to his limits post ts.
He was absent in WCI which was how long?
After all, it's good to see Zoro struggle once again, it was long overdue to see him approach and surpass his limits.