Here's why:
* Generational Exclusives: Pokémon games often feature exclusive Pokémon in each generation. Articuno is a first-generation Pokémon, and Diamond is a fourth-generation game.
* No Direct Trade: Diamond doesn't have any direct trade functionality with the older games. You can't directly transfer Articuno from Gold, Silver, or Crystal into Diamond.
Ways to potentially get Articuno in Diamond:
1. Trading with someone who has it: If you know someone who has an Articuno from an older game, you can trade for it. This would require someone to have access to older games and transfer the Articuno to a Diamond game.
2. Events: Nintendo occasionally holds events that give away rare Pokémon, including legendaries. It's possible (but not guaranteed) that they might hold an event where Articuno is distributed for Diamond players. Keep an eye out for official announcements.
Remember:
* Trading with other players is the most likely way to get Articuno in Diamond.
* Be cautious when trading, as you may encounter scams. Always trade with trusted sources.