Lack of capital to put it simply. Even before the Russian Revolution the USA was acknowledged by Marx and Lenin as the most economically powerful capitalist nation. Seriously the US economy was massive especially after the 2nd World War, take a look at this graph for comparison. (I know some think GDP is a 'fake' measurement but it's really not that bad as long as you don't try to fetishize it) Of course the US built that economy with slavery, genocide and exploitation but those things do tend to make a select few people a hell of a lot of money. The USSR was always behind the US and having to find creative ways to make up that gap.

The USSR by contrast inherited the feudal backwater of Russia, the place with some of the worst infrastructure and industrial development in the world. The Russian Empire was big, but it was also wartorn and full of many nationalities that made it extremely difficult to govern. The Bolsheviks managed to fight a civil war, unify all that land and all those peoples, then valiantly fought off the largest land invasion on world history, then pushed the country into Superpower status and directly opposed the most powerful empire in human history for half a century. That's fucking impressive, and while they did not succeed in destroying global capitalism, they competed well and scared the shit out of the bourgeoisie for a couple decades. Communism allowed that country to punch far above it's weight.

But alone the USSR never really stood a chance in the long term, the USA is massive and also remember that they were fighting with the colonial powers like France and England too. The West's resources only got bigger while the East's continued to fall behind, forced to choose between defense and growth. The USSR mostly had only poorer nations as allies who often needed aid from the Soviets just to stay stable (as most of the countries were ravaged by colonialism and war previously, by fascists or colonizers) but they still managed to aid revolutionary movements in the Global South and some of those still endure (Vietnam, Cuba, etc.)

China was in the same position as the USSR, lots of resources, people, and land but very little capital. Mao, Deng, and now Xi have successfully developed the country and with a combination of foreign investment and homegrown industry they are turning the country into a superpower. This time the economics are actually favoring China in the long term in competition with the USA... it's a much more even fight and the CCP can still press the communism button for full mobilization of the economy against the US if they really wanted to.