The main reasons responsible for the disintegration of USSR were:
Political limitations of the Soviet System:-
I. The Soviet system became very bureaucratic and authoritarian, making life very difficult for its citizens. Lack of democracy and the absence of freedom of speech stifled people who often expressed their dissent in jokes and cartoons.
II. Most of the institutions of the Soviet State needed reforms: the one party system represented by the Communist party of the Soviet Union had tight control over all institutions and was unaccountable.
Economic limitations:
I. Economic stagnation for many years led to severe consumer shortages and a large section of Soviet society began to doubt and question the system.
II. The Soviet economy used much of its resources in maintain a nuclear and military arsenals and the development of the satellite states in Eastern Europe and within the Soviet system. This led to huge economic burden that the system could not cope with.