One major reason was out of control military spending. The USSR could have quietly slashed military spending which would have solved budgetary problems.
Sounds reasonable but was impossible in the reality: the military expenses had been too intervened with pretty much all other parts of the economy and cutting them (even if there was a will to do so) would result in a chaos everywhere. Not that it would guarantee solution of the problems which had been slowly killing the SU like endemic corruption, agriculture falling apart, technology increasingly lagging behind, etc.
It just needed to stop trying to keep up militarily with the West and opt for credible military at minimal cost.
Would not work either. The 1st most probable result would be disappearance of the Soviet Block and the 2nd scenario close to one happening after the fall of the SU which we can see now: "vacuum of influence" is unrealistic and it will be filled by the "West". It looks like even now in a reasonably non-ideological Russia this is causing unhappiness but for the SU this would be a complete disaster.