Final answer:
The COMECON failed due to its inability to support and rebuild Communist Eastern bloc economies, unlike the Marshall Plan. The Soviet Union's refusal to accept aid and its insistence on Eastern European nations rejecting it also contributed to COMECON's failure.
Step-by-step explanation:
The COMECON (Council for Mutual Economic Assistance) failed because it was unable to effectively support and rebuild the economies of the Communist Eastern bloc nations. Unlike the Marshall Plan, which provided significant aid to Western Europe, COMECON faced challenges due to the relative weakness of the Soviet economy, which had high military and space program expenses. Additionally, the Soviet Union's refusal to accept aid from the Marshall Plan and its directive for Eastern European nations to reject it limited the resources available for economic recovery in those countries.