The Ancients that returned from Atlantis after they found out the hard way that the system they spend 10,000 years to get to was already
inhabited by a hostile force and unprepared for military aciton they were both outnumbered and outgunned.
They fought for millenia but were beaten.
Faced with extinction they decided to return to their ancestral home and used the last of their Zed Pee Ems to land, putting aside their advanced
knowledge until the time was right and reverting to a more primitive existence.