Here's why:
* Evolution: Rampardos evolves from a Cranidos, which is only found in Pokémon Diamond and Pearl, not in Platinum.
* No Trade Evolution: While you can trade Pokémon between Diamond/Pearl and Platinum, Cranidos and Rampardos don't have a trade evolution.
To get a Rampardos in Platinum, you'll need to:
1. Trade: Get a Cranidos from someone who has Pokémon Diamond or Pearl.
2. Level Up: Level up the Cranidos to level 30 to evolve it into Rampardos.
Let me know if you have any other Pokémon questions!