Here's why:
* Event Specific: The Ruins of Alph event is a special event designed to be triggered only with a specific Arceus obtained through a particular method. It's not just any Arceus.
* Specific Arceus: The Arceus required for the event is the one you get by completing the "Arceus" game. This Arceus is specifically flagged with the event data needed to trigger the Ruins of Alph sequence.
* Traded Arceus: When you trade an Arceus, it loses its event data. This means that the traded Arceus won't be recognized as the correct one for the event, even if it has the same moves and stats.
Unfortunately, there's no way to "hack" or manipulate a traded Arceus to make it work for the event. You'll need to either:
* Get the specific Arceus from the game "Arceus": The only way to get the correct Arceus for the Ruins of Alph event is to get it by completing the game "Arceus".
* Wait for another event: There's a chance Nintendo might bring back the Ruins of Alph event in the future, but this is uncertain.
I hope this clarifies things for you!