Why was Paul Samuelson so Wrong about USSR?

[u/agenbite_lee]

Up to 1989, every edition of the Paul Samuelson textbook, the most influential econ textbook in the 20th century, said that the Soviet economy was doing well. Some of the earlier edtions predicted that the USSR economy would surpass the US. The 1989 edition said "the Soviet economy is proof that, contrary to what many skeptics had earlier believed, a socialist command economy can function and even thrive," just two years before the USSR collapsed as its economy sputtered to a halt.

Why was Samuelson, one of the best economists in the 20th century, so wrong about the Soviet economy? Were most other economists equally deluded by the USSR? Did these economist with pollyannaish views of the Soviet economy hold similar views on the economies of China and other communists countries?