the visual companion talks about it a little -- that it took a very very long time to even reach the new galaxy, an entire generation was born and died travelling there. I got the impression that the only people left behind on earth were those who wouldn't survive long anyways. the earth was used up -- there was little left to support a population. I figured that once that generation on earth died out, that was it. maybe it's regenerating and once they use up the firefly universe they'll head back to earth :)