Here's why:
* Regigigas is exclusive to Pokémon Pearl: It's one of the legendary Pokémon that are exclusive to one version of the game.
* Trading is the only way to get it: If you want Regigigas in Diamond, you'll need to trade with someone who has a Pokémon Pearl game and has caught Regigigas.
To catch Regigigas in Pokémon Pearl, you'll need to complete the following steps:
1. Beat the main story: You need to have finished the game and become the Pokémon League Champion.
2. Obtain the Regis: Catch the three Regi Pokémon (Regirock, Regice, and Registeel) by finding their respective temples and using the "Regi" keys to unlock them.
3. Go to Snowpoint Temple: Head to Snowpoint Temple, which is north of Snowpoint City.
4. Use the Regis: Once inside the temple, you'll need to place the three Regi Pokémon on the pedestals in the main chamber.
5. Battle Regigigas: After placing the Regi Pokémon, Regigigas will appear and you can battle it.
Good luck catching Regigigas!