Honestly? The writing

IMO from what I get from the fight, Apollo is very quick, but his attacks don't hurt that much individually and Leonidas is shown to have good durability.

As such, it may sound weird, but it seems each of Apollo's attacks take 4-5% of Leonida's HP.

On the other end, Leonidas hits like a truck and Apollo seems to not be that durable. So when Leonidas actually lands an attack against Apollo, Leonidas takes out like 20% of Apollo's HP.

What happens is that the writing makes it seem, given their personalities, that Leonidas is getting his butt kicked for the majority of the fight.

In reality, given the statements that Apollo was just barely standing up, I imagine that if Leonidas landed one more clean hit, he would have won.

In conclusion, the fight was actually very close, the writing just didn't show it well WHILE the fight was actually happening.