I so far would have to say I think that the most likely thing is that we only see swords imbued to show relatively high effort. Like straining against Birdcage or using special moves. Zoro had his swords clad during his last ditch attack on Kamazo, but the same attack is used on Kaido as a warm-up and we don't see it. Obviously he'd want to cut Kaido at least as hard as he cut Killer, but I don't think it was the same effort.
Meanwhile Luffy clads his whole body and it's so taxing that he can't use Haki for a while after. Law immediately realized it was "too much" even without knowing Luffy's capacity in any way, I think because of that.
And lastly, we don't see Kaido clad at all. So I'm thinking we'll see Zoro start attacking with it when Kaido either starts kicking some ass or flexing Haki himself. But I feel like it's a conservative move based on stamina, and so he wasn't cutting lose with Enma.