How do some governments get away with denying their citizens human rights?

In general, governments are very powerful. As long as the military remains loyal to the government, it is possible to retain control of a country even if there is widespread resentment about the human rights situation. If you have the power to kill anybody who disagrees with you, it is then possible to get away with anything. But the history of despotic regimes is that they only get away with this for a certain length of time. Eventually they are overthrown. In the case of the USSR, it took 70 years of human rights abuses before they were overthrown.