Here's why:
* Legendary Pokémon behavior: Legendary Pokémon like Articuno have a unique mechanic in Pokémon GO. Once they flee, they are gone from the encounter, and you cannot re-encounter them in the same spot.
* Limited encounters: You only get one chance to catch a Legendary Pokémon during a Raid Battle. If you miss that chance, it's gone.
However, you can encounter Articuno again in future:
* New Raids: Articuno will appear in future Raid Battles, giving you another opportunity to catch it.
* Events: Niantic often runs events where Legendary Pokémon like Articuno are featured. These events offer more opportunities to encounter and catch them.
So, keep an eye out for future Raid Battles and events where Articuno might appear!