The fundamental reason for the disintegration of the Soviet Union is that Stalin's socialist system has serious drawbacks.
The disintegration of the Soviet Union occurred in the late period of Gorbachev's administration, which was the direct result of Gorbachev's wrong reform route. Communist party, headed by Gorbachev, and the leading group of the Soviet Union were directly responsible for this, and the mistakes in the political reform of the Soviet Union during this period were the most direct and important reasons.
Gorbachev put forward the task of reform immediately after he took office in 1985. Prior to this, the Soviet Union had carried out several "reforms", all of which were of an improved nature and lost the opportunity for reform. In the 1980s, the Soviet Union really faced the urgent need for profound reform. It is not impossible for CPSU to conscientiously sum up the past historical experience and lessons, adopt correct reform strategies and tactics, and make efforts to gradually solve the problems left over from history, overcome various drawbacks in the system, alleviate various contradictions and make the situation better.
However, the CPSU leading group headed by Gorbachev made a series of major mistakes again and again. Under the pressure of anti-communist and anti-socialist forces at home and abroad, they kept retreating, eventually deviating from the socialist road, intensifying various contradictions in Soviet society, causing all-round chaos and crisis, leading to the loss of national leadership and the disintegration of socialist union countries.