There was an explanation to the fact that they didn't colonize: thanks to the erradicator, if they left the planet they'd die. Then the erradicator itself left the planet, so they were all fucked forever. Of course, that story happened over ten years ago so practically no one knows about it nowadays and if someone cares enough about the story to suggest that it should prevent writers nowadays from introducing kryptonian characters when they really believe they should, then that person should be shot in the head.
