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