The main reasons for the collapse of the Soviet Union are
The lack of a professional young bureaucratic apparatus. Most of the officials were in old age - an average of 75 years. But the state needed new personnel capable of seeing the future, and not just looking at the past. When the officials began to die, a political crisis in the country was due to the lack of experienced personnel.
Movement for the revival of the national republics, wishing to develop independently, outside the Soviet Union.
Deep internal conflicts. In the eighties there was an acute series of national conflicts: the Karabakh conflict (1987-1988), the Transnistrian conflict (1989), the Georgian-South Ossetian conflict (started in the eighties and continues on this day), the Georgian-Abkhaz conflict (late eighties). These conflicts finally destroyed the faith of the national unity of the Soviet people.
Sharp shortage of consumer goods. In the eighties, this problem became especially acute, people were forced to spend hours and even days in queues for such products as bread, salt, sugar, cereals and other goods necessary for life. This undermined people's belief in the might of the Soviet economy.
Inequality in the economic development of the republics of the USSR. Some republics were significantly inferior to a number of others economically. For example, less developed republics experienced an acute shortage of goods.
Unsuccessful attempt to reform the Soviet state and the entire Soviet system. This unsuccessful attempt led to a complete stagnation in the economy (stagnation). In the future this led not only to stagnation, but also to the complete collapse of the economy. And then the political system that did not cope with the pressing problems of the state was also destroyed.
The decline in the quality of manufactured consumer goods. Deficiency of consumer goods began in the sixties. Then the Soviet leadership took the next step - it curtailed the quality of these goods in order to increase the number of these goods. As a result - the goods were already uncompetitive, for example, in relation to foreign goods. Realizing this, people stopped believing in the Soviet economy and increasingly paid attention to the Western economy.
Lagging of the standard of living of the Soviet people in comparison with the western standard of living. This problem was especially acute in the crisis of the main consumer goods and, of course, the crisis of technology, including domestic technology. This caused already growing discontent of the population.
Closure of the country. In connection with the cold war, people practically could not leave the country, they could even be declared enemies of the state. Worship of the Western way of life was not approved.
Denial of problems in Soviet society. For a long time, the state completely concealed shortcomings in society, despite their presence. And then at one point, sharply recognized their existence. Belief in communism was again destroyed.
Disclosure of classified materials about the repression and cult of the individual leaders of the country.
A number of man-made disasters occurred in connection with incompetent leadership.
Subversive activities of the United States and NATO countries. NATO countries, and in particular the United States, sent agents to the USSR who pointed to the problems of the Union, brutally criticized them and reported on the advantages inherent in the countries of the West. By their actions, foreign agents split the Soviet society from within.
These were the key reasons for the collapse of the Union of Soviet Socialist Republics. Such a number, the more incredibly acute problems could not be resolved by any successful bill. The collapse of the USSR was an irreversible process. The official announcement of the collapse of the USSR was announced on December 26, 1991. Before that, on December 25, the president of the USSR - Gorbachev resigned.
https://www.quora.com/How-come-the-Soviet-Union-fell-without-a-single-shot-being-fired-considering-it-was-a-superpower
