It wasn't that the government that was unable to make any money off it ... in fact, one of the first steps towards making marijuana illegal was the institution of a pretty hilariously high tax on it. The reason why marijuana was made illegal was that hemp was threatening to infringe on the wood-paper business that William Randolph Hearst had a huge stake in, and he controlled enough of the newspaper and journalism industry that he had the perfect mouthpiece for releasing anti-hemp propaganda ... so he cooperated with a xenophobic government that wanted to further demonize Hispanic immigrants in order to pander to the racism of the time, and turned "cannabis" (a medicine that was accepted by the doctors of the time as valid) into "marijuana" (a "demon weed" that made white women desire fornication with Negroes and Mexicans, and turned white men into rapists and murderers). It wasn't about the government not making any money. It was about a select few private industrial interests shutting a low-cost option out of the paper, textile, and fuel industries in order to secure a hold on their market share, and a government pandering to these interests by appealing to a racist public and creating a way to turn blacks and Hispanics (the habitual users of marijuana at that time) into criminals in order to prevent them from successfully rising in society.