Here's why:
* Legendary Birds are Version Exclusive: Articuno, Zapdos, and Moltres are each exclusive to one version of the Johto games:
* Articuno: Pokémon Silver
* Zapdos: Pokémon Gold
* Moltres: Pokémon Crystal
* SoulSilver is a Remake of Silver: SoulSilver is a remake of Pokémon Silver, so it follows the same rules as the original game.
How to get Articuno in SoulSilver (technically):
* Trading: You can obtain Articuno through trading with someone who has a Pokémon Gold or Crystal game.
* Event Distribution: There have been rare events where Articuno has been distributed in SoulSilver, but these were limited-time events.
Let me know if you have any other Pokémon questions!