Here's why:
* Exclusive to Diamond & Pearl: Shaymin was a special event Pokémon specifically distributed for Diamond and Pearl. It was not available in Platinum, even with events.
* Event Pokémon: Shaymin was only obtainable through a specific event in which players would receive a code to redeem a Shaymin. This event has since ended.
However, you can still obtain a Shaymin if you have access to these methods:
* Trading: If someone with a Shaymin from Diamond or Pearl is willing to trade it, you could potentially get one.
* Pokémon Bank: If you have a Shaymin from a previous game and transferred it to Pokémon Bank, you can transfer it to your Platinum game using the Poke Transporter. However, this requires you to own both Pokémon Bank and the Poke Transporter, and to have access to a previous game with Shaymin.
Remember, it is important to only obtain Pokémon through legitimate methods, and avoid trading with untrusted sources.