Here's why:
* Volt Tackle is a move learned by Pikachu's evolution, Raichu. This means Pikachu needs to evolve into Raichu to be able to learn Volt Tackle.
* In SoulSilver, Pikachu can only evolve into Raichu by using a Thunder Stone. You cannot teach Volt Tackle to Pikachu through any other means, like TMs or breeding.
So, if you want your Pikachu to learn Volt Tackle, you'll need to evolve it into a Raichu first!