I'm currently in public policy school, so hopefully I can try to articulate why I think the policy is the way it is and why it should or shouldn't change. If I can't do this well obviously I need to practice my skills more.
First, I myself grew up with a strong sense of individual liberty, my mom died when I was really young, my dad worked all the time and rarely supervised me, and from about the age of 8 on to my present age of 24 I have had pretty much no one telling me what to do except what I myself thought was right and wrong. I had guidance, but my dad left me a lot of freedom to decide things for myself, he has his own views on religion but never indoctrinated me with any of them, he has own views of politics, drugs, guns, etc. but let me make up my own mind. Because of that, I'm now an adult and I value that individual freedom highly -- higher than pretty much any other principle for declaring the value of something in relation to the ways of government.
Therefore, personally, I believe that not only weed, but all drugs should be legal all the way up to heroin and crack and that each individual person should be responsible for themselves and what they choose to put in their own body and that the government should have no right to interfere in any way (except to possibly educate citizens on the merits and consequences of any possible drug consumption in an intelligent and informative way); however most people and most governments find this view horrific and irrational and I can understand why.
People our age (I'll say 30 and under without intentionally excluding the older members of the forums) grew up in an era much different from previous generations. Our generation places individualism in a much higher regard than community by virtually everyone. In the past a lot of things about your life were pre-determined based on where you came from, who you ran with, who your family was, etc. etc. (group and communal identity). A few months ago before I started graduate school I was teaching math, and I was teaching kids as young as 7 years old who had their own cellular phones - you can hit up the mobile of a first grader now and its normal for kids to have their own phone number.
A lot of people who are older tend to view community and social values much higher than we do today and think that they should be something definite. I am from Placeville, USA we don't do that here that's not who we are and it actually means something of tangible moral value. I personally don't value that kind of thinking at all, but that is the majority held value for most people in all societies today still. I am of the mind that I am some dude and I'll do this or that based on how I think it will effect me and it's none of your fucking business one way or the other. The thing is, that most people who are growing up today are of my type of people, they are hyper-individualized and they will come to voting age and decide that the government has absolutely no business telling me what I can and cannot do when it does not legitimately affect them one way or the other. That is the social/people angle.
The government angle, I see in the same way I see most other laws. The United States (and to be fair to Ho-Jo and others -- Any other Western-style government overseeing a loosely capitalist economy) has every incentive to create laws in such a way that they maximize productivity. Legalizing marijuana in our country would certainly increase consumption of marijuana in our country. We'd certainly have more quality music being produced here, but would we have more widgets being produced at the widget factory if everyone was getting high all the time? Not likely. On the plus side maybe it would reduce stress and overall be a net gain in healthy living, maybe it would replace other more destructive vices and improve quality of life. I don't know, and people who design policy don't have the luxury of seeing all of the positive and negative consequences until after it is enacted. It's not always good enough to say "Look at the Netherlands, they are top tits, let's just copy their policy." In the United States there is a culture of big everything and high consumption, super size me, give me a grande latte, fucking give me a 80" TV that sucks my cock. I'm not really sure if moderation is even in the collective U.S. cultural vocabulary. On top of that you want to say "this drug is now legal, go hog wild." The consequences for a lot of people would probably be very substantially negative.
It is not in a capitalist country's incentive to create laws which decrease its productivity, because it gets tax revenue from productivity, it gets economic strength from productivity, and in our country virtually every aspect of your life is somehow influenced by what you do for money. Unless the net legalized income gain (and it would most certainly be taxed) from legalizing marijuana or (anything else) outstrips the net production loss from encouraging more citizens to actively consume drugs, it is not in the government's incentive to do so.
Eventually, enough of us (strong individuals) will outstrip them (moralizing communitarians) at the voting booths that marijuana will be legalized -- I don't know when but I don't think it will be too far down the line, and I think the same about gay marriage too, that it should be legal, (but a lot of the same arguments would apply in an anti-gay marriage defense of current government policy).
Please criticize any of my claims that don't make sense or are outright wrong. I'm in policy school trying to actually work in government some day -- MAKING THESE TYPES OF DECISIONS FOR A LIVING -- so the way I develop and defend these arguments matters to me quite a bit.