The foundation of American industry via the 'Puritan/Protestant Work Ethic' contributed to a national zeitgeist against Labor unions. But, don't be misled, Labor started int the United States. Since the dawn of industrialization, the US was the world leader for organization and unionization.
There are two leading trends in the rise and fall of American labor, industrialization and systemic prejudice. When American industrialization rose, American labor rose along with it, as did systemic prejudice. When American industrialization started to fall and then collapsed, so did labor, and instead of blaming the flight of industrialization, labor used prejudice to blame someone else: Women in the workforce, blacks being treated like human beings, Mexicans invading our industries, globalist elites screwing the little guy. It doesn't have to be, but American labor has consistently been racist, xenophobic, anti-immigrant, and nationalist. It doesn't have to be, but it always has been.
If you ignore the relationship that American labor has had with white nationalism, xenophobia, and organized crime, then you're missing a major reason why American labor is weak. American labor historically grew when white nationalism, anti-immigrant sentiment, and organized crime was on the rise; It historically declined when immigration expanded and civil & human rights were extended to disenfranchised people. American labor collapsed at the same time that organized crime collapsed in-part because the biggest labor unions in the country were directly connected to the biggest organized crime syndicates in the country. In the 1960s the struggle for labor leadership was a struggle between organized crime leaders.
Track the various rises and falls of the KKK; Track the various rises and falls of organized crime; Track the rises and falls of anti-immigrant xenophobia; Track the rises and falls of nationalist populism... Along side all of them you can track the rise and fall of American labor. On the reverse, if you track progress in civil rights, gender equality, and immigration, those periods usually correlate to a decline in the power of labor in the country. GIven the hostility to immigration, propensity for white nationalism, and a move towards economic populism ... who knows, maybe American labor is about to enter a new golden age.
Today, virtually all labor unions in the US stand against racism and for inclusion. Their members and rhetoric, though, regularly move towards racist rhetoric and towards xenophobia and systemic gender inequality. America is a racist country and its systems are thoroughly racist, and labor is one of those systems. It's a tough legacy to shake. Still, today, though, American labor has a powerful presence in politics especially locally. Local labor unions are still incredibly strong, often the strongest special interest groups within an area and this is why state and local politicians are hungry to get union endorsements. Nationally, though, labor is as weak today as it has been in ~100 years.
We're pro-labor, but my wife has a tired relationship with her union; she feels as though they reward people who have been in the field longer, who already make more money, on the backs of young people who are financially insecure and trying to break in. She feels there's a lot of vested interests that make it more difficult for her or her peers. She can't ever say that to anybody else though because it would be career suicide. Still, though, in time I could see her pushing to move into a leadership position in her union... she's very involved, lots of social connections, but she doesn't dare share her opinions about how she feels like her peers have been screwed by recent negotiations by her union.