The political machines of the time became common in most major cities during the Gilded Age for, in my opinion, two main reasons. First, the cities grew too quickly with immigration and industrialization to be regulated and planned, which invited corruption. Second, the Gilded Age was tailor made for the rich, who could buy influence and power so that Washington DC would look the other way when they built their machines.
Once built, the bosses relied on rigged elections and the immigrant vote to maintain power and rake in the profits of their organization.
Machine politics offered disenfranchised immigrants from all corners of Europe a common denominator...a political boss that would address their needs in exchange for their patronage. City politicans saw the immigrants as a voting base that would solidify their authority within a voting district, and the immigrant saw the politicans as those who had the power to assist with food, shelter, and jobs. The reason that machine politics became the dominating force in the large cities was because that was where the immigrants were. The urbanization of the cities was the result of industrialization; the demand for unskilled labor met an unskilled labor force in the major U.S. cities. Although it would seem that both sides benefitted from this arrangement, too often the powerful political machines took advantage of the unknowing immigrant. All one has to do is look at the political satire of the day, for example the artwork of political cartoonist Thomas Nast. The pictures speak the truth.