I think one main reason we don't use hemp that much for all those things is because of the multimillion dollar corporations that produce oil and everything related to it. These corporations have the government by the balls in most cases, because they provide a lot of jobs and a lot of money to whatever country they are in. Since they are so valuable, governments can't really afford to piss these companies off by making it legal for a resource the oil/plastics/other oil based products corporations can't make any money off of, like hemp. It's really convenient for these companies that marijuana is illegal, and they don't want any of it to be made legal even if its harmless because hemp production will take money away from them.
thats my opinion