Here's why:
* No in-game encounter: There is no location or event within the game where you can encounter Darkrai naturally.
* No trades: You cannot trade for Darkrai in the original Pokemon Platinum game.
* No events: While some Pokemon games had events where players could receive Darkrai, Pokemon Platinum did not have any such events.
To get Darkrai in Pokemon Platinum, you would need to:
1. Use cheats: This would involve modifying your game data to add Darkrai to your game. This is not recommended, as it can affect your save file.
2. Transfer it from another game: You could transfer Darkrai from a Pokemon game where it was obtainable (like Pokemon Diamond, Pearl, or HeartGold/SoulSilver) via a Nintendo DS wireless connection, but only if you have a friend with that game and the Pokemon.
Remember, using cheats or glitches is generally discouraged in Pokemon games, as it can take away from the intended experience.