The formation of national labor unions in the late 1800s was mainly a response to

In the United States, the formation of national labor unions in the late 1800s was mainly a response to the incredibly poor working conditions in the factories that came about as a result of the Industrial Revolution.

I believe the answer is: the incredibly poor working conditions in the factories that came about as a result of the Industrial Revolution.

Prior to the establishment of national labor unions, the companies were known to give unfair wages, dangerous project site, and really long hours for the workers. This made a lot of labor workers suffer under their circumstances, and the national labor unions was created to address this problem.