re: Why is Marijuana illegal?

You can thank William Randolph Hearst, Andrew Mellon, and the DuPonts for it. Later it would be Nixon, whos disdain for anti-war protestors and hippies alike, put the final nail in the coffin in a legal sense with the Controlled Substances Act; all after he had been advised that it was mostly harmless and should be legalized. LSD met the same fate at this time as well.