Here's why:
* Heart Scales are not found in the Underground. They are found in specific locations in the main game, usually as rare drops from Pokémon.
* The Underground is a separate area. It's not a part of the main game world, and items you find there cannot be transferred to the main game.
To get Heart Scales in Pokémon Pearl, you'll need to:
1. Defeat certain Pokémon. Some Pokémon, like Chansey, have a chance to drop Heart Scales when defeated.
2. Explore specific locations. You can find Heart Scales in certain areas, like the Valley Windworks.
3. Trade with other players. If you're lucky, someone might be willing to trade a Heart Scale for another item you have.
Remember, Heart Scales are quite rare, so be patient and keep searching!