What are the main reasons for the illegality of Marijuana in the USA?

I'm looking for a few well elaborated reasons over a bunch of under
explained ones.  I'm also very interested in the places you will find
this information, and the methods that you used to find them.

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Clarification of Question by hometack-ga on 14 Nov 2004 21:30 PST

Would an increase in the list price promote a good answer here?

One reason that is sometimes cited is that marijuana is a "gateway
drug" that often leads to the use of other drugs.

Like most (if not all) anti-marijuana propaganda, this theory has been debunked:

http://www.upi.com/view.cfm?StoryID=20021202-030109-4556r

Here's an excellent history of the illegality of marijuana:

http://blogs.salon.com/0002762/stories/2003/12/22/whyIsMarijuanaIllegal.html