Why Is Marijuana Illegal? The History Of Cannabis In The U.S.

Cannabis has a long and complicated relationship with U.S. law. The plant was used and accepted as a medicine for many years before it was criminalized in the early 20th century. Hemp was also widely used throughout the 18th and 19th centuries for textile purposes. Both George Washington and Thomas Jefferson grew hemp. 

So  If, as a medicine and as a textile, cannabis proved to be a valuable resource, why did it become illegal?