Here's why:
* Pichu is not a naturally occurring Pokemon in SoulSilver. It was introduced in Generation II, but not as a wild encounter.
* You can only obtain Pichu through breeding. This requires having a Pikachu that knows the move "Sweet Scent" and breeding it with a compatible Pokemon (like a female Pikachu).
So, if you want a Pichu in SoulSilver, you'll need to get a Pikachu, teach it Sweet Scent, and then breed it with another Pikachu.