Well, the USSR had the most natural resources and a high population (289 million in 1990), so these factors would offset the economic inefficient of the socialist system. It is true that before the bolshevik revolution, russia was just a poor agrarian country. By the 1950s, it was an industrial powerhouse. So many view communism as the reason why that occurred. Eastern europe was incorporated as the "eastern bloc" and it was plundered after ww2. Machinery and capital were sent off to Russia
. Cuba became dependent on the soviet union when it came to trade, and it wasn't the only them that were dependent. So when many countries depend on you for their survival and you're the "big dog", it is easy to stay afloat. It is important to note that while the USSR was proficient in machinery and heavy industry, consumer goods were rather scarce. Food shortages were not rare and stores barely had food in them, but it was the only thing they were used to and known. When the USSR was no more, Cuba almost collapsed. But they made economic reforms in 1993 which sorta stabilized the country. So it is no longer full socialist, but rather a mix of capitalism and socialism but closer to socialism.
Cuba has also been exposed to the west far more than Russia, so they're less hostile to capitalism. Russia has been stagnant since the 90s so many russians miss socialist times.