Here's why:
* Thunderstones are used to evolve Pokemon like Pikachu into Raichu. In Pokemon Yellow, Pikachu is a special case. It's your starter and will not evolve into Raichu unless you trade it to another game, like Red or Blue, and evolve it there.
* There is no way to obtain Thunderstones in Pokemon Yellow. This is specific to the game's design.
To evolve Pikachu in Pokemon Yellow, you'll need to:
1. Trade it to a friend with Pokemon Red or Blue.
2. Use a Thunderstone in Red or Blue to evolve it into Raichu.
3. Trade the evolved Raichu back to Pokemon Yellow.
Let me know if you have any other questions about Pokemon Yellow!