Here's why:
* Shaymin was originally distributed in the events for Pokémon Diamond and Pearl. These events were held in 2008 and 2009.
* Pokémon Platinum was released in 2009, after the initial Shaymin events.
* While some players may have obtained Shaymin through trades or other means, there was no official event for Platinum.
If you want to get Shaymin in Platinum, you would need to obtain it from another game and trade it in.