First of all, it is not clear that all of these movements failed for the same reasons. If I had to pick something that they all had in common, though, I would argue that these movements failed because Americans in general did not support them and because the political system was at least somewhat rigged against them.
All of these movements were movements of relatively small and relatively weak segments of American society. The farmers’ movement and the Populists were both made up mainly of farmers. Farmers were no longer the majority in the United States by this time and they were fairly isolated in rural areas. This meant that they did not have the sympathy of large portions of the urban population.
The labor unions were, of course, centered in the cities. They did not make up a majority of the population either. More importantly, they lacked support for at least two reasons. First, many of the members of the unions were immigrants at a time when the United States was experiencing a fair amount of nativism. Second, many Americans felt that unions were too politically radical, an impression bolstered by such things as the bombing at Haymarket. Thus, none of these groups had a tremendous amount of popular support.
In addition, the American political system was at least somewhat rigged in favor of the rich. Political candidates were picked in “smoke-filled rooms” by party bosses, not in actual elections. The parties were influenced by rich donors who could provide them with money. In addition, the US Senate was, at this time, not directly elected by the people. Instead, it was elected by state legislators, thus making it easier for big businesses to influence the selection of the senators. This gave rise to the notion that the Senate was, in essence, bought and paid for by big businesses.
For these reasons, the reform movements lacked the popular support that would have been necessary to overcome the advantages held by the rich in the American political system of the time.