Here's why:
* Darkrai is an event Pokémon: Darkrai was only obtainable through specific events in the past. These events involved receiving a special code or connecting your game to a specific website.
* No in-game methods: Darkrai is not available through regular gameplay, breeding, or trading within Pokémon Platinum.
What you can do:
* Try trading: You might find someone willing to trade you a Darkrai if you have a rare or valuable Pokémon to offer in return.
* Use a save editor: You can technically obtain Darkrai using a save editor, but this is considered cheating and can corrupt your save data.
Important note: Obtaining Pokémon through unofficial means can violate Nintendo's terms of service, and you may risk being banned from online play.