For example because he needed to know the weakness of king, fought king now longer time, which was also fighting even before longer etc. pp..
King was not one shot or mid diff, zoro was pushed to his borders again, that’s why he could unlock AdCoC now.
Funfact:
While Kaido there one shots Luffy, Kaido couldn’t do the same with Kid even before. Since Hawkins explains that Kid and Killer had a longer fight against Kaido.