if you want to attribute the prohibition of hemp merely to standard oil you are really missing a lot of the story. Hemp is what paper used to be made out of. Awesome paper that lasts forever. Then someone invented a way to make paper out of wood. Then Hearst bought that patent and a bunch of trees and wood pulp paper mills. Wood paper turns yellow due to the acid content. Eventually it disintegrates, in a few years all the books from the last century will disintegrate.

Anyhow Hearst's bankers nephew was the head of what would become the DEA and he was the one who pushed for cannabis prohibition, he made the reefer madness movie for example.

Hearst did his part by demonizing cannabis in his newspapers, often writing the propaganda personally.

Once banned he basically had a monopoly on paper. Hearst's yellow paper is the origin of the phrase "yellow journalism"

The other side of the story is what keeps hemp and cannabis illegal, if we look who funds anti legalization efforts we see it is the drug, alcohol and gambling industries. They know cannabis is competition and cuts directly and significantly into their bottom lines. Legalizing cannabis will cost those industries tens or hundreds of billions of dollars. Also the beet industry opposes legalization. My guess is that they are afraid farmers won't bother growing beets anymore when they can make a lot more money growing hemp.