I doubt that much though went into it actually. It seems as if logical reasoning has nothing to do with drug laws.
I think cannabis and psychedelics were outlawed as a backlash against the rapid social changes of the 1960s and 70s. It was an easy scapegoat for politicians and their ilk to beat up on to garner support from voters who did not like the new direction the country heading at the time (e.g., civil rights, environmentalism, anti-war movement, college protesters etc).
Why did these drugs stay illegal then? I think there are several reasons why drug laws have not softened even as evidence became available that they weren't as bad as previously portrayed.
First, there has been a lot of hyberbole spouted off as the official government stance on drugs, and a lot of money has been spent. Many people have been incarcerated, more than anywhere else in the world. At this point, no politician wants to be the one to admit that this was all a waste, in terms of money and peoples lives (those incarcerated included). It's analogous to the Vietnam war, no one wanted to be the one to ''lose the war'' after so much has been spent, and so many have been wasted, so the war went on even though it was obviously a failure. Goes to show where our leaders intentions lie, they care more about ''winning'' than the public well being.
More importantly, there is a lot of money at stake with the war on drugs. The groups that are benefiting from prohibition will fight tooth and nail to keep the status quo in place. what was the DEA's budget last year? If drugs were legal, they would all be out of a job. How many people in jail are there for drugs? The private companies that handle our enormous prison population would loose fortunes,and towns that have prisons would loose jobs. How much property was seized by police departments under forfeiture laws? That's a huge source of free money for them. how much of any given police department's efforts go to drug enforcement? budgets would be slashed and officers laid off if they didn't have to enforce such laws. And those groups, particularly police officers, carry a lot of weight on public opinion. Voter eat up whatever they say, and no one ever votes to cut the amount of police officers in their area. Surely a lot of money would be put into fighting any legislative change proposed.
And finally, Public perception is terribly skewed on the topic. There have been decades of misinformation put forth as fact about drugs, in schools and in public service announcements. The ''news'' media loves sensationalist stories about drugs. moral panic pieces are cheap and easy produce, especially compared to actual investigative journalism. And most people don't really care much about the issue. Most people don't use illegal drugs, and I think a lot of them view it as a removed problem. They think of drug users as shady criminals operating in the inner city and not as just regular people.
Ultimately, as the drug war keeps on chugging, and more of the electorate have a family member caught up in the legal system because of it, people will eventually reconsider these policies. until then, just try to inform people about the realities of drug use and users, and donate to a pro legalization group if you can.