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
To answer this question you need to ask why did Gorbachev come to power in the first place? It is obvious to everyone that he is not a communist and that he probably never was.
He came to power because by the 1980s the USSR had lost its way so much that it was struggling to justify its existence. The mindset of 'outcompeting' the west has its roots in the 1920s but really became front and centre of the Soviet mindset in the 1950s after Khruschev became leader. This meant that the Soviet government instead of focusing on revolution and anti-imperialism, instead started focusing almost purely on economic growth and geopolitical dominance.
Due to the underdeveloped nature of the country, bad economic reforms, and just poor management of the economy in general, the Soviet economy was never able to surpass that of the US despite indications that it would be able to do so. The lack of other major allies and the decline in global popularity of Marxism-Leninism in from the mid-1970s onwards damaged its ability to project power and the country became increasingly weak.
When you define success as being more powerful in a bourgeois imperialist sense than the capitalist west and then fail at that goal, it is no surprise then that people start looking at adopting their systems and ways of thinking in order to achieve that goal, this is where Gorbachev comes in. When he opened up the channels of communication to everyone people began communicating their long-suppressed views. Views that namely were, questioning the existence of such a political Union, with an extensive apparatus of repression and rigid economic and political thinking. For many of these people, these institutions had not delivered on their promises of improving the lives of people and keeping them safe.
The USSR was never meant to be a socialist equivalent of the Russian Empire, however that is what it pretty much ended up becoming. The emblem has a globe on it for a reason, it was supposed to be like the EU, a union of socialist nations that was constantly expanding and did not favour any one nation, granting autonomy to all. Instead because of the way it was constructed, especially after world war 2, it was pretty much defined as Greater Russia and Russians were increasingly given a privelaged position. This caused the alienation of other nations, especially the Baltic states, which had strong national identities and were absorbed into the union through a dubious referendum which no one believes represented the true views of the peoples it polled.
In terms of social progress, there was not much after the 1930s. Women's rights were better than the west, true, but they did not keep progressing, and most of the visions of the revolution were forgotten about. Men continued to dominate most higher-level positions and women continued to be relegated to domestic work on top of their careers. Homosexuality was illegal from 1931 onwards and afaik no attempts were made to decriminalise it until after the USSR fell. In terms of the economy, the advance towards communism stopped after the 1940s, every economic reform after that was a regression into more capitalist relations.
There were also few efforts to reform the prison and repression systems, despite the fact that they were deeply loathed by the population and were moreso run to protect the state from unrest rather than to suppress class enemies and resolve contradictions. The fact that in the 1960s and 1970s you could find circles of Marxist Leninists at universities being persecuted for arguing that the government was not fulfilling Lenin's ideals set out in State and revolution tells you all you need to know. A punitive rather than transformational approach to justice was taken, and this imo was one of the biggest flaws of the Soviet Union, it created a lot of unnecessary suffering and prevented the society from moving towards more progressive social forms.
Ultimately when you have a bunch of problems, remove/suppress the revolutionary mindset to dealing with them, and promote the liberal mindset instead, you end up with Gorbachev.
This? This is a good post.