Here's why:
* Dawn and Dusk are mirrored games: While they share many elements, the Digimon available in each game are different.
* Impmon is a Dawn exclusive: Impmon is a unique Digimon that only appears in Digimon World: Dawn.
If you're looking for a similar Digimon in Digimon World Dusk, you might consider getting a Terriermon or a Renamon, which can both evolve into strong Digimon.