The setting itself is tailored such that the IoM is the only reasonable form of government, and there is more freedom tolerated in the IoM than our current western regime. Total homogenization and control over every aspect of life is neither desired nor pursued by the Imperium. Most planets are left to operate in any way they see fit as long they pay their taxes and don't mess with chaos or xenos, although some interaction with xenos is tolerated. The Imperium puts all its effort into protecting its borders and citizens.