It was the stagnation in the 1970s where the centrally planned economy couldn't adapt to consumer goods, light industry, and technology where they fell further and further behind. Various incentive problems exasperated things. Things would have been much worse in the 70s if high oil prices didn't mask these underlying problems, allowing the USSR to obtain hard currency to import goods.

Don't get me wrong, communism was bad. However, there were periods where it appeared to look superior. The quality of life in North Korea was "superior" to the south until the late 70s (much of this was backed by soviet subsidies, etc), but the optics still existed. There was a similar, though much more brief, period where east Germany had a faster growing economy than the west. There is a reason the Berlin wall was built in the 60s and not earlier.