There is always a reason and most of the time it has to do with money.
In 1937, the Marihuana Tax Act of 1937 was passed in the United States. It levied a tax on anyone who dealt commercially in cannabis, hemp, or marijuana. The reasons that hemp was also included in this law are disputed—several scholars have claimed that the Act was passed in order to destroy the US hemp industry, with the primary involvement of businessmen Andrew Mellon, Randolph Hearst, and the Du Pont family.
One claim is that Hearst believed that his extensive timber holdings were threatened by the invention of the decorticator, which he feared would allow hemp to become a very cheap substitute for the paper pulp that was used in the newspaper industry. Modern science suggests that this fear would have been unfounded. Improvements of the decorticators in the 1930s, machines that separate the fibers from the hemp stem, could not make hemp fiber a very cheap substitute for fibers from other sources due to the fact that the long strong fibers are only found in the bast, the outer part of the stem. Only about 1/3 of the stem are long and strong fibers.
Another claim is that Mellon, Secretary of the Treasury and the wealthiest man in America at that time, had invested heavily in DuPont's new synthetic fiber, nylon, and believed that the replacement of the traditional resource, hemp, was integral to the new product's success.
The Marihuana Tax Act of 1937 was ruled unconstitutional by the Supreme Court in a unanimous verdict decided in Leary v. United States, and ultimately superseded by the Controlled Substances Act of 1970.
Hemp was used extensively by the United States during World War II. Uniforms, canvas, and rope were among the main textiles created from the hemp plant at this time. Much of the hemp used was cultivated in Kentucky and the Midwest. During World War II, the U.S. produced a short 1942 film, Hemp for Victory, promoting hemp as a necessary crop to win the war.