Feimei Cosmic Country had always declared that only one divine guardian was protecting it, but the actual number of guardians was known only to the King and the high-ranking members of the Congress. Even Medea, who grew up with Elizabeth, was unaware.
As the King of a country, knowing too many secrets, the Federation Government wouldn't allow a retired King to wander around freely. For a retired King, they offered only two choices, one was to go to the Outer Domain, which was actually a veiled form of exile. The other was to stay in the Feimei Cosmic Country and become a Guardian, which was essentially imprisonment in disguise.
Of course, other than picking from retired Kings, there were other ways to select Guardians. For example, retired members from the Military Department, from captured interstellar pirates...