Because Celebi is associated with Gold and Silver...just look at the GS ball' name and is green which could be based off the line of how the Aztecs valued Jade more than Gold and Silver and could lead to RSE as it's a precious stone rather than crystal being a more generic term...also by picking Suicune they made it more important than either Entei and Raikou...I would've perfered Raikou too over Suicune...Pokemon Cintrine.