Here's why:
* Shiny Pokémon are incredibly rare: The odds of encountering a shiny Pokémon are 1 in 4,096.
* Pikachu is not available in the wild: In HeartGold and SoulSilver, Pikachu is only obtainable through trading or by catching one in the Safari Zone (which has a separate, lower shiny rate).
How to get a Shiny Pikachu in HeartGold and SoulSilver:
1. Trading: The easiest way to get a Shiny Pikachu is to trade with someone who already has one.
2. Breeding: If you're lucky enough to get a Shiny Pikachu, you can breed it with another Pikachu to try and pass down the Shiny trait.
3. The Safari Zone: The Safari Zone offers a slightly higher chance of encountering a shiny Pokémon, but the chance is still incredibly low. You would need to spend a lot of time searching to find a Shiny Pikachu.
Keep in mind: Even if you follow these tips, finding a Shiny Pikachu is mostly down to luck!