Here's why:
* Lake Guardians: Mesprit, Azelf, and Uxie are the Lake Guardians, each residing in a specific lake in their respective games:
* Mesprit: Found in Lake Acuity (Pokemon Pearl)
* Azelf: Found in Lake Valor (Pokemon Diamond)
* Uxie: Found in Lake Verity (Pokemon Platinum)
* Cresselia: Cresselia is also a legendary Pokémon exclusive to Pokemon Pearl and Platinum.
To catch Mesprit and Cresselia, you would need to play Pokemon Pearl or Platinum. You can't transfer them from another game into Diamond as trading is not available between Diamond, Pearl, and Platinum.