We actually don't really know. We do know that two hooves (like sheep and goats) are better for gripping on really rough terrain.
Single hooves aren't better for speed either - cloven-hooved antelopes run very fast too. It might be that a single hoof is better if you want to be both fast and big (although wild horses are very small compared to domestic horses, they're still bigger than most deer).
Truth is? We don't actually know.