The true answer is going to sound terribly mundane, but it's probably just because that's the kind of thing they evolved from, like most animals.
We've got jaws the way we have, because we came from things which had that body plan. It'd be the same for them. They evolved from something which has multiple sections, whereas we came from things which had just the two. Their purpose is clearly to shred flesh into tiny pieces. If something evolved from, say, ants, you'd probably get the end result having sideways-closing mandibles, too.
Alternatively, they might be like squid, where their brains are a doughnut-shape around the mouth and they need to make whatever they swallow a certain maximum size or risk brain damage. The actual part food seems to go down does appear rather small.