Internally: Bureaucratism (AKA Careerism)
Externally: US Pressure in the form of the Arms race and economic decoupling which divested crucial economic production capacity from consumer goods to heavy production and armament, eventually causing the standards of living to widen too much in comparison to the west.
How much one or the other was more important is hard to know, but the two death blows came in the form of the Sino Soviet Split, and the Soviet Afghan war.
Red Pen or the Marxist Project has a vid on soviet cybernetics, where they show that the issue boiled down to bureaucratism causing the death of the project.