Why aren’t volbeat and illumise fairy or electric type?
Because they don’t need to be? Fireflies are real insects that exist in the real world, so there’s not really anything about them that needs to be explained by adding another type. I mean, sure, I think there are arguments for either of those, or Fire for that matter, but if you wan’t a more specific answer than that I’m afraid you’re going to have to ask Game Freak; I have no special insight here.
Pokémaniacal 