The most common conspiracy I have heard is that the Dupont coporation was primarily responsible for weed becoming illegal since many of the products made with petro chemicals and their byproducts, especially nylon, can be made cheaper, more safely(ecologically) and with less energy consumption with hemp.
This story dates back to the 30's or 40's when pot was first outlawed. Hemp could conceivably eliminate our dependence on foreign supplies of crude oil thus eliminating our need to ship it, process it, burn it, etc etc. This would also eliminate a huge source of profit for the oil industry. I have yet to see any concrete evidence on this one much like the whole UFO thing at Roswell. Difficult to prove or disprove but I'd be surprised if this has not played some part in the continued resistance of the feds to distinguish between hemp and marijuana and therefore, keep both plants illegal. Commercial hemp is pretty much impossible to get a high from and it is only logical to conclude that if we can breed varieties with high levels of THC, we can also breed varieties that are more conducive to industrial uses. But the DEA and other fed agencies flat out refuse to acknowledge a difference.