The distances and travel times between colony worlds made it impractical for the central government to exert tight control over these remote, tiny populations, so they were left mostly on their own. As long as the expected resources flowed back to Earth, the colonies were allowed to govern themselves as they saw fit.
Most of these worlds adopted democratic governments of one sort or another, and the frontier in general had a spirit of egalitarianism that was long ago lost on Earth, if it had ever truly existed there.
Democracy on the rim was not perfect. Some of the worlds established enhanced levels of citizenship, usually preferencing those born on the world over those who emigrated from Earth. Others produced their own odious politicians not unlike those on Earth. But generally, the colonials were a rugged, self-reliant breed, resentful of the intrusive government they'd left behind, and highly resistant to the development of anything remotely similar on their new homeland.
When colonies were tiny and starships slow, this arrangement worked without any problems. But as the importance of the colonies increased and populations grew, the central government on Earth began to interfere more and more in day-to-day colonial affairs. The colonists resisted these attempts, and tensions slowly increased. The nearly constant warfare with the other powers kept this problem from becoming severe by focusing energies and resources on defeating the enemy. But the discontent remained, and it was increasing.