As much fun as big conspiracy theories are this one is a load of crap. While i'm sure big businesses like the oil industry, alcohol industry, tobacco industry and pharmaceutical companies have a vested interest right now, they're not the original cause or reason for it's continued legal status.
Let's work backwards. Pot was made originally made illicit in 1937 thanks to the hard work of Angslinger. Well laws don't get passed over night. It took a lot of hard campaigning for it to get passed, So lets go back 5 or so years. What was happening in the US in 1932? A: The great depression. When the banks collapsed, lots of business flopped and consequently lots of people lost their jobs. These newly unemployed flooded the welfare offices. This flood of people demanding money is economically very dangerous for governments. So who can we stigmatize to lighten the load? Immigrants! who are these immigrants? To answer that we have to go back another 10 or so years. Early 1920's; WWI just ended and during the war the US industrialized on a scale never before seen. This started an economic boom. But to help feed that boom a large labor force is need. So where can we get lots of cheap labor? A: IMMIGRANTS! HEY! look at that, the country just to the south of us has just such a labor force.
Ok, now let's jump back to early 1930's. lots of people out of work and we need to lighten the load. Now we know who our superfluous population is. In order for us to stop them from taking our scarce money and jobs we need to deport them. How? take something they're doing but the average citizen doesn't do and make it illegal. What is that something? Cannabis.
The fact is, the southwestern states pressured fed to make cannabis illegal so they would have ground to deport this excess population and alleviate the pressure on their welfare offices and therefore free up some money for other things. The man in charge, Anslinger, could only make a law stick by convincing congress and the general public that cannabis was dangerous and that the only solution was prohibition. His attempts at demonizing pot and the fact that prohibition (thx temperance movement) was popular at the time made it possible for cannabis to be outlawed. The reason why it remains illegal is historical baggage. Most of our beliefs come from our parents and those around us. If everyone is saying pot is dangerous and evil, in general we'll belief pot is dangerous and evil. We won't question it and will furiously defend it because it is "common knowledge" or "common sense". Th reason politicians are so slow to changing the laws is because that is what they grew up hearing. That is a strong held belief that won't change anytime soon.