Apart from racism and prejudice, another thing that was a big factor in outlawing marijuana was the production of hemp. The owner of a paper company, and a friend of Nixon, was concerned the production of hemp, which is arguably better than paper, would cut into his profits and run him out of business. So he lobbied his friends and started a slew of campaigns against marijuana, leading Nixon to agree, I’m assuming because it was an excellent opportunity to target black communities and the hippies, both of which he despised.