It is easier to understand why it is so hard to legalize marijuana, when one looks at the sheer volume of anti-marijuana propaganda which was disseminated by the vast Hearst publishing/entertainment empire, which inspired a thousand copycats.  This campaign reached near hysteria levels, much like the USA's dubious flirtation with Prohibition, the 18th Amendment (BTW the only one to ever be repaeled.)

Prohibition did not work in the 1920's and it does not work now.