Below are the main reasons
Economic Stagnation: by the 1980s, there was zero economic growth, development of the black market for consumer goods, could not produce enough grain, rationing of food, consumer goods were inferior, poor use of resources (Pipeline)
Military Spending: 15-25% of GNP, Americans spent only 5-7%, too much on guns, not enough on butter.
Political Stagnation and Corruption: Party officials lived a privileged life, they did not want reform when it was most needed.
Ideology: Command economy, complete centralized planning, individual initiative is replaced by quotas and threats.
Nationalism: 50% of the population was not Russian, the non-Russian ethnic groups would fuel the break-up in combination with economic problems and the unwillingness of the new Russian gov’t to turn guns on its own people, like they had before.