I honestly believe that the main reason people still continue to oppose drug policy reform is ignorance. Many people have false notions about drugs and their effects on individual users and society, largely thanks to decades of propaganda and societal notions that get drilled into people's heads generation after generation. While sure, some people are never going to change their minds no matter how much you try to educate them, the vast majority of people would be willing to stop and reconsider their opinion if you show them the unbiased evidence that indicates that our current drug policy is not ideal - not for the individual user, not for the economy, and not for global society.
I think it's also very common to underestimate the importance of drug policy reform - the article summary here is a great example of that. The thing is that drug policy reform isn't just about the rights and freedoms of individual users; it's not even about the potential tax revenue we could generate. The fact is that we are never going to eliminate the demand for drugs, and as long as drugs remain illegal, criminals are going to reap those bountiful profits - which are then used to recruit young, impressionable thugs, buy weapons and ammunition, bribe government/military/police officials, and generally cause violence and chaos. Mexican drug cartels and the Taliban are both a direct threat to our national security, and both derive a great part of their funding from the drug trade. We stand to increase our own national security by legitimizing the drug trade, alongside all the other almost uniformly positive benefits that arise. Those billions of dollars will now be flowing through the legitimate economy, where they will generate taxes, create jobs and businesses, and more. Not only that, we will save tax money on law enforcement and prisons. Crime across America would be reduced - not just the obvious, but also because otherwise non-criminal drug users would no longer be forced to consort with real criminals in prison, and also because there would be no more crimes incidental to drug use or trade, such as addicts stealing to get their next fix (when was the last time you saw a tobacco smoker stealing cars to buy a pack?) Addicts would be able to seek real treatment for their problems without being stigmatized as a criminal, and the rate of injury and death related to drug use would drop significantly since drugs would be regulated for dose and purity.
Well, I'm yammering, and I'm sure you agree with what I'm saying - but the point is that the rest of the world needs to be told all of this. We could gain so much even just from decriminalization (just see all the positive results in Portugal!), but full legalization is truly the way to go. Unfortunately, I don't believe I'm going to see full legalization in America in my lifetime - maybe of cannabis, but not of any other drug. It's unfortuante, because as a result of our drug policy, our country is causing great harm to both itself and to the rest of the world. But I can only do my part in trying to educate people, to bring us just a bit closer to the day when we'll finally have the sense to say that our drug policy just doesn't work.