Jor-el realizes that the DCU is a bad place for Krypton, some folks are going to try to explode it. He realizes that under a yellow sun, Kryptonians will be awesome. He is also aware of Daxamites having the same potential.
Thus, he invents a portal to switch universes. He convinces the governments of Krypton and Daxam to go along. They aware that the MU has its share of menaces but looking into DC future, doesn't see the same number of endless universe cleansing crises. So off the two planets go to a unoccupied system with a nice yellow sun to charge up the population - who learn to use their powers. The planets also use their science to arm up.
How do they fair if the usual MU menaces show up? Galactus, Phoenix to eat their sun, the various empires - Skrull, Kree, Shi'ar, Thanos, etc. In the Great Darkness Saga, Darkseid decided to take a hike when 3 billion pissed off Daxamites were heading for him.