The Hush of Expansion
The local star group is denser than in our universe, meaning that, while there is no Faster Than Light travel, it only takes between weeks and months to travel between two neighbouring star systems. Travelling across the whole of the human diaspora is still a large undertaking, requiring many, many months if not years to achieve.
Due to the spread of the diaspora, no single overall authority has had a chance to emerge. In the majority of cases, each solar system has structured itself by whatever particular ideology initially colonised its planets. Earth’s technological progress at the time of the dispersal was approaching a post-scarcity society, and as such when the call for volunteers to go and establish new worlds went out it was eminently possible, and indeed sensible, that entire social groups went to establish a new life where they could be free from persecution. Adherents to a common religious creed or practitioners of a lifestyle model all took the chance to find a place in which they could live out their chosen lives.
Naturally, the technological advances of Earth itself were not always possible to send with the colonies – or in many cases they were simply prevented from benefiting. Therefore the frontier worlds, while having been settled for two or three generations, are often woefully behind in their technology. See Technology for more details.