According to National Geographics Taboo show, the Cotton farmers are the ones that heavily lobbied against Hemp because they were scared it could put them out of business in clothing and other fabrics, so in the early 1900's they managed to get their politicians to pass a bill "protecting their way of life."

But either way it boils down to about the same, some group lobbied against it because it would put them out of business, the corrupt politicians agreed and made it all illegal for their benefits. I firmly believe the only reason they won't make it legal now is because the Federal government makes money on it each year.