What started as a desire for freedom from the Old Earth powers; for unity and equality; for the ability to colonize away from war, eventually stalled.
The United Commonwealth of Systems sprang up from a handful of the earliest of Earth’s colonies who’s ideals then spread across known space. More and more colonies excitedly joined the movement, throwing off the rule of the Old Earth powers.
However, with the passing of time, and the influence of human nature things slowly, inexorably changed. Bureaucracy grew. Resource rich colonies deciding to go it alone or not join the movement looked increasingly desirable. Some planets were taken by force. Others were heavily leaned on until they joined. Colonies of other powers were annexed and wars ensued.
In the end, we find ourselves in the current situation. The UCS is the dominant power of the Galaxy enforcing its will on the interstellar empires of the Old Earth superpowers, and other nations who have formed. Space exploration is generally illegal without a permit from the UCS. Colonization is strictly controlled with severe sanctions placed on any attempts at unsanctioned planetary settlement.
On the other hand it is fairly straight forward to bid for colonization charters, and provided taxes are paid and certain regulations are followed, the UCS will leave governments alone. Out on the fringes of human space it is possible, and actually quite normal to live life with barely the slightest acknowledgement of the existence of the UCS.