"The reason is that Type: Null was constructed to synthesize the strengths of various Pokémon, enabling it to adapt to any situation."

So it required Pokemon with a lot of type coverage to make it adapt to any situation. Using the theories that Type: Null is a combination of the final starter evolutions and Rockruffs final evolution, and the leaks that state they would be Grass/Ground, Fire/Dark and Water/Fairy, the only types Type: Null needs to cover are Fairy, and Normal. And in the very likely event that Rockruff evolves into a rock/steel type, the only type left to cover is normal, it's own type.