This is all my own opinion and therefore as a result of years of consumption of novel chemicals provided to us by nature is likely to be a bit ranty and quite wrong or it may well be right, I will allow anyone to make there own decisions and conclusions but here are mine.
The Problem with legality of any mind altering drug/chemical/plant is down to several main points. The first being the duration the substance has been known and used, this is closely tied to who uses it and why and will it lead to chaos.
Take alcohol as a starting point. This drug has a history too long to write down, it is used buy many cultures and peoples through out the world and accepted by many religious groups. Therefore it is widely accepted taxed and ok for governments. The people who use it do so to relax, forget or are addicted and this is good for society as people generally do the same predicable things when consuming it. They will become loud, sociable leading to uncontrollable, sexual promiscuity and usually/unfortunately for some violence. But the governments get large amounts of tax from this substance so put up with the negative and it is a good pacifier (you have a bad week in work or you life goes wrong for what ever reason you get drunk and take your mind of it and have a "good time") when you hangover is gone usually the need to make waves/change situation is gone with it and you carry on with your normal existence. The argument to make this one illegal is void as could you imagine if no one could have a drink after work the nation would go crazy leading to chaos, which is what the governments try to avoid. Also anyone can brew alcohol, so near impossible to police if it is banned.
Now if we look at say psilocybin mushrooms, found worldwide used for probably longer than alcohol as they just grown and all you have to do is eat them. So age is a factor by right we should be looking at these like we do baked beans or vodka but this substance was not used by the correct cultures. In western society the predominant religion is Christianity. Back in the day missionaries would travel the unknown world spreading the "good word" but at the same time they denounced the religious practices of natives as pagan and against god. Now unlike current society the church formally would make the rules and the conservative views and deal a swift and heavy blow to destroy teachings and practices of these pagan cultures, they went as far as moving Jesus' birthday to December to cover up and assimilate European pagan festivals such as imbolc or the winter solstice. The fear and loathing passed down generations of policy makers and they have prevailed to modern day law. Let’s say we now make these legal the country goes out wandering the countryside eating the mushrooms they find. Inevitably people will eat the wrong ones and fall ill/die this scares the governments. Also then public become unpredictable, free thinking takes over and suddenly people don’t need the dogma of modern society the system falls to chaos. There are no taxes paid for the substance as it just growths outside and you cannot police people who consume it as they are likely to not listen as your view is not stronger than their own.
Then we should look as LSD. Only about 80 to 90 years old, public image of this is your typical hippy type person with "far out" views. It’s a new drug so no history to relate to other than what has been created in living memory of the nations. It was made aware to people during the counter culture movements starting in the late 50s, massive in the 60s and 70s and on through to present day. No religious use when looking at traditional belief systems. So by the conservative view of society it is neither taken seriously by enough people/drug companies to be worth exploring scientifically nor taken by enough people to be widely accepted by those who do not take it like with tobacco or alcohol. Also again there is the chaos factor, military experiments found it too unpredictable to use for anything (unsurprisingly love drugs and war don’t mix). Public experimentation produced things like Woodstock which made the governments afraid, large groups of people with alternative views to the state are at the end of the day enemies or as they like to call then now "terrorist". So they stamped down hard and made LSD obscure and difficult to get hold of and ridiculed advocates.
Really it all boils down to one issue in my mind. Take your typical drug user. Usually the outsider when in school, involved in groups of people who don’t take the education system seriously. Usually they don’t start on LSD it would usually start with smoking cigarettes, then alcohol and on to weed then once the use of illegal substances become normal exploration begins the user may get hooked on bad drugs and fall out of society or use good ones and chose to stay of the edges of society. This means the people who regularly use these substances don’t put them selves in positions to make to relevant changed to law. Politicians may have smoke but not inhaled during university or college. If they got into anything more serious they would have not got into politics any further due to the major bummer it really is, too much paper work lies and to quote a technical term "bullshit". The "good" politicians are everything the public want. Stable, confident in the public eye, does the right thing at the right time and most of all actually works hard. These people at school age are the nerds, swats and goodie two shoes. No offence meant but they are not your typical drug fiends. They steered clear of distractions when young and this mind set stuck in later life, they remember the druggies in school and see drop outs and no hopes so drugs must be bad and the law does not get changed because its there to stop this kind of social misconduct.
It’s a sad day to realize that the situation is unlikely to change in the life time of anyone who reads this but we must take stock of the situation and be grateful. If it was all legal then we would be kicking off over the cost/supply/demand issues of a world high on the same trip and sobriety would be the counter culture. I think that is a far scarier prospect than the unlikely event of being arrested for picking some mushrooms or eating small squares of paper.