I agree with "ike", but I also like to add that previously on the planet Lorien, Setrakus Ra used the Harvestor to harvest the Entity, aka Lorien that resided in Mogadore, in an attempt to grant Legacies to non Garde. If the Cepans told their Garde about the Pheonix Stones to reawaken the Entity, and did just that, then Earth would be in huge trouble. The Mogadorian's huge influence on the Earth's governments as well as Setrakus Ra's technology will basically destroy the Earth's ecosystem by harvesting the Lorien inside, effectively turning it into a barren place like Mogadore.
"Back thousands of years ago, Mogadore once had the Entity residing in it as well. When Setrákus Ra was banished from Lorien, he hid away on Mogadore. Using the energy within that planet, he progressed Mogadore with science, technology, and magic. He called it the Gift. Over hundreds of years, Setrákus Ra harvested so much of this energy that the planet grew absent of it completely. The environment collapsed, and the flora and fauna died out. All that remains of life on Mogadore are Trueborn survivors or Vatborn creations of monstrous beasts and Mogadorian soldiers. Without even a spark of this energy left to awaken, Mogadore cannot be restored (Lorien Legacies Wikia)."