Answer :
The government's central planning lead to a huge, complex bureaucracy that discouraged efficiency and lead to indifference. Moreover, collective farmers had no incentive to work hard.
There were several things that could be mentioned as reasons for the weakened economy of the Soviet Union during these decades. As the country modernized, their planned economy did not have the proper structure to account for the deep changes that modernization brought about. Moreover, the volume of decisions that needed to be taken became overwhelming for its government, and bureaucracy became very slow. Finally, the War in Afghanistan greatly damaged the Soviet economy.
