Not going to say anything but this: don't bring religion into this thread. Religion and pokemon=something reaaallly bad about to happen. >.>;;;
Aaaanyways.. Not everyone flocks to serebii.net and knows Arceus exists, Nick. Just.. mostly everyone.

That, and if they don't officially announce it, noone would know it was definatly in the 12th movie or figure out when and how it'd be distributed.
Also, did you know noone knew Arceus existed until some random player was using metronom on one of his pokemon, and used Arceus' signature move "Judgment"? When reported to the pokemon community, people were baffled and confused what pokemon had the move, so they delved into the game's data and discovered pokemon #493: Arceus. Pretty lucky discovery, eh?