I'd argue that it has always been, on some level, looked down on because of how land is viewed in Socialist thought. Early immigrants to the United States came because of the ability to own land in the new country; at the time in Europe land was owned by the wealthy. Even though some of our founding fathers had very different views on the role that land should take, the end result was still individual ownership. The biggest far left movement to grow in America (Individualist Anarchism) reflects on the idea of land as pivotal to personal freedom.

Of course, the growth of big business in the mid to late 19th century softened many of who Marx would call the proletariat to the far left. This relationship grew up around the Unions that were used to fight for workers rights. As far as the disillusionment with the far left goes, I can think of a few factors over time that caused it to weaken.

• The Unions actually won a lot of what they were fighting for while at the same time were neutered by many government laws and regulations. A very different environment was created when Unions became more tempered. Unions sought to disassociate themselves with the far left to better represent the workers within the restraints of American political thought. Those who refused to dissociate with the left (Like the Wobblies) were persecuted alongside leftest during the many red scares.

• When the immigrating population shifted from west European protestants to non protestant and east or non Europeans they were labeled as unhygienic, poor, and politically radical. While this was not always untrue it helped to create an Us vs Them mentality, with the left being them.

• The political left deals with equality on a broader scale than just class. Socialism would force Americans to face racial and gender inequalities that made them just as uncomfortable then as it does today.

• The center left constantly drains the life out of the far left. Every time a political movement succeeds you have people who say "Alright, thats it. We're done." People either reach the point where they are not comfortable with change or the change that benefits them has been won already and they don't see any sense in fighting for nothing. This is largely what happened in the 60's. The radicals of the 60's are just left of center today. Same happened after the New Deal reforms, many people were satisfied with liberalism.

• Fear also plays a major role in people disassociating with the left. Whether its fear of anarchist radicals that put bombs in the mail or fear of Russian Communism, people were scared of the left for a lot of the 20th century. It was even used by republicans for political gain when they used the conviction of Hiss to paint Truman as soft on communism. Misinformation plays a lot into this too, people saw any form of socialism to be related to the USSR and therefore against American ideals.

• The growth of the middle class also hurt the far left because the left does best in times of hardship and struggle. In times of opulence, especially when the wealth relatively more equally distributed, conservatism is more appealing because it tends to be static in nature. For a lot of people static means comfortable.

• There were two major efforts by the government to root out the far left. The first was kicked off by the palmer raids as fear of Bolshevism and Anarchism rose, due in part to terroristic attacks by Anarchists. The second came after WWII with McCarthy's trials. Both arrested, deported, or blacklisted many with connections to the left. Including some of my favorite singers.

I honestly couldn't point out a single point in time and say "There is when the far left lost its appeal."

Edit: I did this late at night and just remembered about it a moment ago. I made some crucial edits from my phone but will add sources and improve it a bit more when I get home. I'm so sorry mods. Thank you for your mercy.