Cuz Zoro never fully used Enma against King:
I'm referring to people that said King's dura power is in his wings only. While he didn't regulate as much of his haki into enma against King, were are just talking about a slightly less hakified version here, and King no sold it.
Meanwhile Kaido had three huge Dragon Twisters at his side, and Zoro DT had to clash, counter, and dissipate all three while leaving a huge gash on Kaido.
Plus we have King (Oda's) own words saying he has a durability power beyond, when Zoro asked if his durability is like The Dragon and Dinos.