Sorry for the long post, but you should take the time to read this because the more people who realize this the faster we can work on ways to counter-act these factors, if it is possible at all.
I don't think cannabis will ever be legalized beyond the state level unless a very very large number of citizens join together and aggressively confront our anti-cannabis government. There are 2 broad, STRONG reasons why cannabis will be tough to legalize:
1) ECONOMY
The tobacco industry would decline, as more people would realize that--if they're going to inhale the harsh combustion products of a plant--it might as well be something that gives hours of euphoria rather than a useless 5-minute rush.
The pharmaceutical industry would decline, because the majority of their products are intended for symptom-relief: because of cannabis' ability to relieve a wide array of symptoms with out many negative side effects, it would replace many side-effect-ridden, potentially unhealthy pharmaceutical relief-products.
The alcohol industry would perhaps be the most damaged by cannabis. Given the choice between cannabis or alcohol as a recreational drug, I think most (if they were smart) would choose cannabis. Alcohol is addictive, hepa-toxic (liver), neuro-toxic, depletes the body of essential vitamins, and it's effects are generally not that euphoric, not to mention the hangover is sickly hell. Cannabis is non-addictive, non-toxic in any way except maybe to the lungs (but that's based on ROA), it is actually neuro-protective in some ways, there's not much of a hangover, and the high is relatively more euphoric. As ironic as it seems, the alcohol and tobacco industries are huge fundraisers for those phony "Above the Influence" commercials that are more funny than they are persuasive: using cannabis as a scapegoat makes their horrendous/destructive products seem more legitimate.
In addition to these^ 3 enemies of cannabis, you also have it's illegality economically tied into law enforcement. Because cannabis is involved in a large portion of criminal drug cases, there is a high number of police/LEO that would be laid off because of the decrease in work that would result from legalization. Also, like other drug cases, cannabis cases generate revenue for the government in the form of property seizures, fines, prison stock value, etc. However, I think this money would be exceeded by the profit from taxation of legal cannabis.
2) CULTURE
This is often the more overlooked--but very influential--aspect of the cannabis-ban. The LSD ban is a good precedent for explaining this. The main reason government outlawed LSD was because it was having a profound effect on the culture of it's users that went against the grain of capitalist consumerism. The US government has always been, and continues to be a corrupt entity. It conspires against it's own citizens, stealing financial resources/power from the middle and lower classes in very subtle ways while it is virtually a puppet controlled by the hands of the economic top 5% class minority. The continuation of this agenda depends on the oppressed citizens constantly working and consuming while being distracted/sedated from the deceit, political propaganda, and mind control pushed by the government. We are kept distracted from the real socio-political oppression with junk television, the false promises of the "American Dream", conformity, and economic competition (in which we end up fighting against ourselves rather than the upper 5% ).
In the midst of the "Turn-on, Tune in, Drop Out" lifestyles of the counter-culture of the late 1960s, government realized that this counterculture had become impervious to government bullshit. When LSD placed the most vulnerable part of the human mind--the ego--on the "back-burner" of consciousness, the mind was liberated. Without the naivety of the ego, these acid-heads were more sensitive to the truth of the larger picture: they started asking powerful questions and could no longer be easily deceived or sedated by government authorities; they "saw right through" the oppressive government agendas and realized how much of a bully government was. This "awakening" presented in the form of people abandoning the spiritually-empty "cookie cutter" lifestyle of never questioning authority, getting married/having several children, greed, the purchasing of unnecessary luxuries, all of which directly benefit the upper class minority. They traded this for a life of living-as-opposed-to-working, nature, community/sharing, spirituality, introspection, and self-discovery, all of which are threats to capitalism.
Although certainly not as powerful as LSD, cannabis effects have more commonalities with psychedelic drugs than any other drug category. It can influence all of the above LSD-induced cultural changes to an extent. A dramatic increase in the number of cannabis-users would likely lead to a revival of a similar counterculture, only in this day and age it would be much harder to suppress once it gained momentum because of how much more-educated our current generation is compared to the 1960s. I think most can agree that the cultural effects of cannabis are not consistent with mainstream/conformist culture. The government is all about using mind-control and programming strategies as a way of preventing the overthrow of those (outnumbered) in power, and the better they do this, the more they are given "contributions" (paid off) by said wealthy people/groups.
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And to think, all of these^^ socio-economic animals grew from the initial anti-cannabis effort of a long-ago greedy farmer who felt threatened by the economic potential of hemp products, so therefore spread exaggerations and/or lies like: cannabis makes people crazy and will destroy their brains and ruin lives. Give me a fucking break!! I was high when I wrote all this, so that should further prove my point about the above-mentioned anti-sedation of cannabis. --Peace.