Here's why:
* Arceus' Location: Arceus is located in the Hall of Origin, which is only accessible after obtaining the Azure Flute.
* Azure Flute Requirement: The Azure Flute is necessary to open the entrance to the Hall of Origin.
* Arceus' Appearance: Arceus only appears inside the Hall of Origin after using the Azure Flute.
To catch Arceus in Pokémon Diamond, you must:
1. Obtain the Azure Flute: This is obtained after completing the post-game storyline and speaking to the Lake Guardians (Uxie, Mesprit, and Azelf) at their respective locations.
2. Travel to the Hall of Origin: This is located on the top of Mount Coronet.
3. Use the Azure Flute: This will open the entrance to the Hall of Origin, allowing you to encounter Arceus.
Without the Azure Flute, you will not be able to access the Hall of Origin and therefore cannot encounter Arceus.