Economical strength to maintain Cold War was gone. Through the late 70's and in the 80's, the Soviet block went from stagnation to deep economic decline where problems with a centrally planned economy became evident. Shortages were widespread, investment into industries lagged, technology became obsolete, utilities collapsed, and infrastructure deteriorated. Energy crises was quit common through 80's and Eastern block was willing to open up their economies to the West. The reforms in Hungary and later in Poland and USSR laid foundation for market oriented policies.
Centrifugal forces within the Soviet Empire led to disintegration of it. The system was build in postwar Yalta structure that maintained Soviet presence regardless what local people wanted. Once the states challenged this order, USSR could either impose stricter military presence or let it go. Countries like Poland or Hungary decided to leave hostile Soviet