Because the event that May 1st commemorates is a Chicago police riot involving massacre of labor activists, and the subsequent framing and execution of several anarchists during the struggle for the eight-hour day. The American government would rather forget. http://en.m.wikipedia.org/wiki/Haymarket_martyrs We still have commemorative protests but they either end up small and inconsequential (cause everyone's at work) or they fall subject to police repression. Americans' union membership is about 1 in 8 as mentioned in the article. This is a significant decline from the historical peak of 1 in 3.A little off-topic, but why is it that Labor Day in the US is not celebrated on May 1st as in much of the rest of the world?
Also, as a non-US resident, how common is it to be a member of a union? Are you in a union?