This is a very political topic, that is difficult to give an objective answer to, because much of it is shrouded in propaganda, personal opinion, and plain difficulty in establishing the facts. I would suggest by reading said essay by Einstein, since it literally answers your question. One major factor that plays into the decline of the popularity of socialism in the West is of course The Cold War, during which there was very widespread suppression of socialist thought both domestically in the US and abroad. On the milder side of this we have McCarthyism, on the harsher - the US involvement in coups aimed at overthrowing socialist regimes.

Another reason is the USSR itself, as it promoted a specific version of socialism, and it often suppressed competing versions (Orwell seems to have been a socialist, but opposed to Stalinism specifically). Of the norm for intellectuals there was to be socialist (some of the cyberneticians of that era are really, really interesting, i.e. Viktor Glushkov is my fave). The fall of the soviet union was also seen as a blow to socialist thought through the 90s.

Finally, we are looking at things back, through the lens of our very modern post-Cold War viewpoint, that has naturally filtered and whitewashed the presence of such intellectuals. MLK is a great example of such whitewashing, as he was openly socialist, there are radical movements such as the Black Panthers, and there are plenty of thinkers outside the United States. There was the entire Third world movement, founded on anticolonialism and socialist thought, and figures like Castro, Allende, Sankara, Nelson Mandela were all very influential thought leaders in the second half of the 20th century. Ho Chi Minh and Mao are also influential in the Third World.

Today we have some fairly popular socialists as well, although not that much to the forefront. The closest to a superstar socialism has is probably Slavoj Zizek, a quirky and charismatic philosopher that you should definitely have a look at. There is also the late David Graeber, who is actually an anarchist but as a lot of anarchists, has a lot in common with criticizing capitalism.

Finally, some resources: A lot of the things all these people I mentioned above wrote is available on the web, so I suggest you give them a look as well, although it is a little dense at times. Einstein's essay is a good introduction. If you want to go into the nitty gritty, have a look at the "Jakarta Method" by Vincent Bevins, which is a really interesting and thorough journalistic investigation of the history of US involvement with the suppression of socialism in the third world. My little thingy lately has been Cybernetics, as it seems to have offered a technocratic socialist vision of the future, and many people involved within it were left leaning - Andrew Pickerings "The Cybernetic Brain" is a beast of a book, but really interesting. And if you want to dig into actual documents - have a look at "Fundamental Principles of Communist Production and Distribution", which is a little earlier (30s) but has a large section that shows the disagreements socialists in the west had with the USSR model.