While I don't fully understand the legality behind marijuana laws (my studies focus on criminal and Constitutional law), the right to bear arms is absolute and upheld by the Supreme Court for the individual right to bear them. However, the Court does recognize and allow the State to regulate the sale and ownership of them as to not be unusual or dangerous for the time period of common ownership and sale.

If I recall correctly, marijuana was made illegal through grassroot movements and heavy lobbying by the timber industry (here in the US) to remove them as a competition for the production of goods. It also doesn't help that a good percentage of incarcerated folks are in there for marijuana charges and with a strong for-profit prison system... it can be hard pressed to change.