Oda did a bad job at explaining this fight, but I think the take away was suppose to be that both King and Zoro were nearing the end of their stamina due to fighting Marco and Kaidou prior to their battle. King probably couldn’t fight for much longer in Flame Mode, just like Zoro couldn’t use Enma much longer. They both mutually opted to end it with a final clash.
The Seraphim are coming into the fight fresh, which is a bigger issue since the SHs are trying to escape egghead and being stalled out by the Seraphim is problematic.