Title states all. It can be multiple reasons or just a few. I've twiddled to down mainly to 3 reasons, "roughly" that is.
The Stalin personality cult that would subjugate various Soviet leaders to being wild conspiracy theorists and untrustworthy of themselves, their inner groups, intelligence, other leaders of the USSR, etc.
The inability for the Soviet Union to give more independence or political freedom to it's satellite states, and freaking the fuck out when states weren't following the strict set of guidelines from Moscow, (also party leadership changing the internal politics and Moscow relationship of it's satellite states every time a Soviet Leader died/changed their mind on how to operate it's states, Belarus comes to mind.)
Finally, the economy, and the Soviets too fraught with conspiracy to adopt to the global economy when the world started to surpass them on many economic fronts, along with a bloated military budget.
These are my reasons, I akin this degradation like a large column of marble representing USSR and the issues that toiled the USSR like many hammers and chisels, some are bigger than others but ultimately no one hammer or chisel brought an end to the first great socialist experiment. Thoughts?
-7DeadlyFetishes
Ussr was sadly doomed from the start. Cpsu / cprsfr was put in the position of trying to build capitalism and then socialism on top of a war ravaged feudal nation with, forget outside help, but constant attack from the most powerful nations in the world. If it could have been saved it would have been to (somehow) end the cold war and make peace with the west Ala China but the China USA peace was only really allowed to happen bc it was a thorn against the Ussr. So yeah it was doomed.
This was probably the greatest factor more than anything. The would never have gotten to make the same peace China got, and China got lucky by us having to do the war on terror tbh