It is incorrect to state that the Knights of Labor were a failure or to suggest that the American Federation of Labor was only modestly successful, as neither statement is true. To that extent, the above response is also incorrect.
The Knights of Labor were more properly known as the Noble and Holy Order of the Knights of Labor. It was organized by Uriah S. Stephens in 1869 and was modeled after the Masonic lodges; in fact it was designed to be semi-religious with secrets shared only by its members. The Knights led a number of successful strikes against the railroads which made them hugely popular, in fact their membership reached 700,000 by 1886. Thus it is hardly correct to suggest that the Knights were a failure. However, the association of the labor movement with anarchism following the assassination of Czar Alexander II made unions unpopular, and membership in the Knights declined.
The American Federation of Labor (AFL) is still in existence, and is a hugely effective advocate for workers. In the early twentieth century, it joined with the Congress of Industrial Organizations (CIO) to form a united organization, the AFL-CIO. The AFL was founded in 1886. It worked for immediate improvement of worker conditions rather than for long term political gains. Its founder, Samuel L. Gompers is said to have worked for:
bread and butter in the here and now rather than pie in the sky in the sweet bye and bye.
The AFL did not eliminate unskilled workers as the above response suggests; but was more successful at organizing skilled workers beginning with the Cigar makers, a union of which Gompers was a member.
The truly radical organization which sought to organize all workers was the International Workmen of the World, commonly known as the "Wobblies." The Wobblies saw the AFL as too conservative. An IWW organizer, "Big Bill" Haywood, called Samuel Gompers
a conceited, petulant, and vindictive squat specimen of humanity,
They worked for a destruction of all government and the creation of a new classless society. The radicalism of their ideas made them very unpopular. The IWW led only one successful strike (compared to several for the Knights of Labor) and their association with Communism brought about their demise. Ultimately, Bill Haywood was forced to leave the country. He fled to Russia where he married and was buried in the Kremlin Wall as hero of the worker's revolution.
The Knights of Labor failed largely because they were too radical while the American Federation of Labor had some amount of success mostly because they were much more moderate.
The Knights of Labor were quite radical. They wanted to abolish the capitalist system. In addition, they represented both skilled and unskilled workers. These things made the general public dislike them and gave them less leverage when bargaining with employers.
The AFL was more moderate, wanting only better working conditions and wages within the system. They represented only skilled workers. These things made them more acceptable to the general public and gave them more leverage over employers.