The US and USSR were equally powerful, playing the same game, with the same population. Ideology vs ideology. Capitalism won by a mile.
Because fundamentally the US isn't driven by an all-encompassing ideology. Capitalism doesn't dictate how every citizen live their life. It only describes how the economic system will work. Anyone inside the system is free to create their own enclave. If you want to move to Oregon to live on a commune, no capitalist commissar is there to stop you. Starting a capitalist enclave in a communist country would have had to sent to the gulag.