Here's why:
* Pain Split is a move that can only be learned by breeding. It's not available through level-up, TM, or any other method.
* Dusknoir doesn't learn Pain Split through breeding either. It's not one of the moves it can inherit from its parents.
Therefore, the only way for a Dusknoir to learn Pain Split is through breeding with a Pokémon that knows the move and can pass it down.