Because Oda is inconsistent as fuck with visibly demonstrating Haki.
We already know Enma was draining Zoro, and he was matching its Haki drain. From 1033, before 1035 when Zoro beats King:
Zoro: "What do I do? I can only keep appeasing it if I keep going all out with my Haki! But that could kill me! No... this is how it's meant to be!"
Zoro couldn't stop Enma's Haki drain, he was using Haki the whole time. Oda just didn't draw it in until the finishing panel, because, once again, visually inconsistent.