Here's why:
* Thunder Stones are for evolving Eevee. In Pokémon Lake Version, Thunder Stones are used to evolve Eevee into Jolteon, not Pikachu.
* Pikachu evolves differently. Pikachu evolves into Raichu by using a Thunder Stone, but only in the main series Pokémon games. In Pokémon Lake Version, Pikachu doesn't evolve at all.
Therefore, you can't use a Thunder Stone on Pikachu in Pokémon Lake Version to evolve it.