For better or worse, union are particularly weak in the United States. This was not always so. Why unions declined is not limited to Reaganism which merely accelerated a trend already present in the data. The difficulty is to explain this trend which is for example only present in some other countries and nowhere as pronounced.
Emin Dinlersoz and Jeremy Greenwood explore whether this has to do with the distribution of income, at least in the US. Indeed, over the past century and a half, union membership rates followed an inverted U-shape, while the income share of the top 10% did the opposite. Greenwood and Dinlersoz think that both can be explained by the evolution of skill-biased technical change: basically, while the assembly-line was the main means of production, unskilled labor garnered a higher higher income share and unions were strong, but both decline since as information technology became important. Nice story, but I wonder whether it can apply to more observations (i.e., countries). Also, I wonder whether the timing of events works out. Indeed, the ratio of of unskilled to skilled workers went into a tailspin starting in 1945, while union membership started decreasing only in 1955 and the income distribution started getting more skewed in the 1980's.