How come they're aren't any rock, ground, dragon, ghost, fighting, poison, flying, bug, and steel Eevee evolutions?
Game Freak hasn't made them yet. That's really the only answer. If the pattern continues, and we get a new Eeveelution/Eeveelutions every second generation, we should be getting more Eeveelutions in Gen 8. But, that's just a theory.