I figured I’d ask here since you comrades know history and are on talking terms with reality, unlike a lot of stuff that is available to read online. I am really looking for a short answer, although I know there were many factors playing out over a long long time. Just the bullet points, if you please.
Edit: Thank you for these awesome answers, a lot of exactly what I was looking for and a lot of new directions to explore. Y’all really are the dope-ass bear B-)
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.