The answer as to why they were not informed about this before the mission began, is because the higher-ups believed that there were several spies in their army, just as much as they had their spies in the armies of the other human empires in Omega.
Some of the soldiers were fast to pick up on this, and others were somewhat slow.
If the higher-ups had decided to inform them before they left for this expedition, then it was highly possible that whatever spy present, would have informed their masters, telling them of the secret opening of the secret grotto.
Eventually, this blows everything up and reduces the chances of the parties involved from being able to monopolize. Even after the fact that only talented individuals were chosen to participate, there were still possibilities that some of them were from the enemy camps.
