The USSR fell. This had two major effects on those that came of age in a world without it.

First, we grew up thinking of Russia as an oppressive capitalist dictatorship instead of an oppressive communist dictatorship. The freedom and prosperity that were supposed to accompany capitalism never arrived. This disproved two of capitalism's major selling points.

Second, the USSR no longer monopolized leftist though. Between WWII and the end of the USSR, leftist groups in the US were systematically infiltrated by the American Communist Party, which was itself funded and operated from Moscow. Ideas that didn't align with the USSR's vision (like gay rights) were rejected. Since most leftists had no interest in bringing gulags to America, this decimated the movement. Without it, alternative visions like democratic socialism or Maoism are becoming more common.

For a look at how much leftist organization was warped, check out this article: https://www.theatlantic.com/magazine/archive/1944/08/richard-wright-communist/618821/