Earth-that-was got used up. Became an uninhabitable rock.
Those that were left had no choice but to move out beyond the reaches of known space and find a new home. A star system was found with hundres of possible new Earths. They were terraformed and colonized. Some became rich and flushed with the new technologies. While others not so much. The central planets joined together to form what became known as the Alliance. They decided all the planets had to join them under their rule.
There was some disagreement on that point.
A war broke out between those for the Alliance in the Core and those opposed to it out on the Rim. After the Unification War, many of the Independents from the Rim who had fought and lost drifted to the edges of civilization. As far from Alliance control as they could get. Out on the fringes of the system people struggle to get by with the most basic of technologies. A ship will bring you work; a gun will help you keep it.
A captain's goal is simple: find crew, find a job... keep flying.