Companies such as Monsanto and DuPont (not to mention a host of others) are among the ones that were glad to see hemp made illegal back in the 1930's.
Any hemp-based economy is a threat to centralized industrial concerns. With the invention of the hemp decorticator in the 1910's (the cotton gin of hemp) by George Schlichten, hemp's potential as an industrial crop to replace petroleum was unlocked.
Large, centralized industries have no interest in seeing their monopoly threatened.
You can easily dig around for more sources, but I've attached the wiki link, which has some information about fuel usage and many other uses. From what I can remember it's about as viable as biofuels from corn (maybe a bit better) but still not really production quality. Hemp's biggest advantage is the strength of it's fibers.
Anything that can be made from a hydrocarbon can be made from a carbohydrate. If hemp got the same subsidies, protections and considerations from the government that petroleum does, I suspect it would be cheaper than petroleum in the long run.
Any hemp-based economy is a threat to centralized industrial concerns. With the invention of the hemp decorticator in the 1910's (the cotton gin of hemp) by George Schlichten, hemp's potential as an industrial crop to replace petroleum was unlocked.
Large, centralized industries have no interest in seeing their monopoly threatened.
Remember, hemp is illegal, not just marijuana.