Here's why:
* Rotom is a special event Pokémon. It wasn't available through normal gameplay in Platinum.
* Event Pokémon often have separate entries. Their data isn't integrated into the regular Pokédex, as they are not part of the standard game's obtainable Pokémon.
To get Rotom in Platinum, you would have needed to:
* Participate in a specific event: Nintendo would have hosted events where players could receive Rotom through special distribution.
* Trade with someone who obtained it from an event: If you have a friend or someone online who has a Rotom from an event, they could trade it to you.
Unfortunately, these events are long gone and no longer possible to access in Platinum.
However, you can catch Rotom in future Pokémon games like Black and White, where it is available through regular gameplay.