Here's why:
* Pokéwalker Data: The Pokéwalker itself doesn't store the Pokémon data. Instead, it communicates with the game cartridge. When you start a new game, all the game's data is reset, including any Pokémon left in the Pokéwalker.
* No Way to Retrieve: There is no in-game mechanism to retrieve a Pokémon from the Pokéwalker if the game data has been erased.
What to do if you left a Pokémon on the Pokéwalker:
1. Start a New Game: This is the only way to get a fresh start in HeartGold and SoulSilver.
2. Consider it Lost: Unfortunately, the Pokémon you left on the Pokéwalker is gone for good.
3. Learn from Experience: Remember to always take your Pokémon back from the Pokéwalker before starting a new game!
Remember, the Pokéwalker was a fun feature, but it was limited in how it interacted with the main game.