The unfair advantage is over other hunters. Hunters would prefer to hunt by daylight, for the most part. If light-hunting at night were allowed, that would be

The unfair advantage is over other hunters. Hunters would prefer to hunt by daylight, for the most part. If light-hunting at night were allowed, that would be the only way to take the best game, because as the comment you're responding to notes, it's much easier. So everyone would be forced to do the thing they don't want to do, if they want to take a deer at all. That's a bad equilibrium, what we call "not sporting".