There are numerous reasons for this:A Foul Tragedy
In These Times magazine, November 2005
There are more folks in prison for marijuana than for violent crimes.
1) Marijuana is a branch of the hemp family, and Big Oil doesn't want to be hemp farmers. As long as marijuana is considered a scheduled drug, industrial hemp will also remain illegal.
2) Marijuana is easy to grow, and Big Pharma wants to remain one of the only two industries legally allowed to profit from it. As long as its possession by the unlicensed is criminally prosecuted, they will.(Of course the drug cartels, and the black market, are an acceptable part of this business model.)
3) Big Prison is the only OTHER industry allowed to profit from marijuana, although these profits are incidental to its legality, or the lack thereof.(Of course the drug cartels, and the black market, are an acceptable part of this business model, as well.)
4) Big Fed, otherwise known as the DEA, has far too much pride invested in the "War on Drugs." to go back now. Should marijuana suddenly become de-criminalized -or made entirely legal- Big Fed suddenly looks like a stone fool. They can't be having that, even though we've already repealed prohibition once, because it didn't work.
It still hasn't worked - for the American People. Working just fine for the above folks though. Don't see it happening anytime soon, but willing to work to make it happen eventually.