I remember this being discussed in a documentary a long time ago (I can't remember the name). A few other points suggested in documentary:

* Hemp/Marijuana was a popular source of fiber and competed with cotton. Cotton requires fertilizer and pesticides (marijuana does not), so the petrochemical industry were keen to squash it as a competitor. Interestingly they claimed it was temporarily legalized during WW2 to allow for rope production.

* Marijuana was also associated with sexual activity (and with both Mexicans & African-Americans). Another reason to portray it as a substance that was immoral.