I suspect that one of the reasons is the experience with medical marijuana in some states, in particular California. There getting a marijuana prescription was so easy that dispensaries became a large distribution channel for recreational users.

This process provided arguments to opponents of medical marijuana, who claimed that it is just a “pretext for stoners”. As a supporter of both medical use of marijuana and derived substances, and of legalization of recreational use, I think that a clearer separation between the two would benefit all users and provide a better legal environment.