I am all in favor of broad cannabis legalization, but there there is something to the gateway theory.

Most users of harder drugs indicate past use of marijuana. Additionally, marijuana gives many their first taste of doing business with drug dealers and 'breaks their cherry.' When they decide they want to try something else they have already gained experience locating dealers and engaging with them. Legalizing cannabis helps here because its users won't engage with dealers to score, they'll go to the store and buy a regulated product.