Because Randolph Hearst back in the 20s controlled the wood based paper industry and the newspaper industry. Hemp was viewed as a viable paper source back then. Marijuana is derived from Hemp. So the greedy bastard Hearst used the newspapers he owned and paid off the movie industry to create a marijuana scare across the Nation so that the import of hemp would be banned. A good example of Heart's manipulation is the movie Reefer Madness. This adverse publicity and sccare caused a huge public outcry against marijuana prompting its recreational to be unlawful.
Anti-marijuana proponents today cite illogical arguments (e.g. marijuana is a gateway drug to heroin) as a rationalization to keeping it illegal. It's really so stupid and sad.