People often obey a gevernment out of habit. Obviously habit is not something a new government has on it's side.
People hate a government which hurts them. Because people had time to adapt to old laws (murder illegal? work as potato farmer rather than assasin!) they are more hurt by old governments than new ones.
Whenever a country is 'liberated' either by external or internal forces, the liberators make all kinds of promises to all kinds of people. Some of these promises at least will be broken, causing members of the new ruling class to turn against eachother.
And often new governments lose the services of the most experienced police and propaganda men who worked for the old regime and our thus less able to defend themselves than established ones.
