From general points of view:
Basically more people are made better off by laws against stealing than by laws against drugs. In fact, it's surprising that we have strong laws against drugs because almost no body, except a few power hungry regulators, and drug cartels, and drug companies, are benefited by laws against drug usage.
- The market tend to take care of things when force or fraud is not involved. Basically laws against consensual acts are not necessary. That's because if drugs are really bad, consumers will automatically not use it. Okay this may need a bit of more explanation. If drugs is really harmful there is no serious need to outlaw it. People would use it less.
- Laws against stealing, however is necessary. That's because even though you are made worse off when someone steal from you, your consent is not needed. Hence, most potential thief victim would want laws against stealing. Stealing is harmful but you can't just choose not to be thief victim.
- Societies as a whole is benefited from laws against stealing. It's for the same reason why mafia have laws against snitching. The mafia as a whole will be better off if they have such laws than if they don't. Any analysis from prisoner dilemma can show this. When stealing is more profitable than producing, in the absent of laws against stealing, people would steal from each other, and everyone would tend to be worse off.
- Because many drugs that governments prohibit are safe drugs like MDMA. Very few people die due to MDMA per 1000 users compared to cigarettes. The reason why MDMA is prohibited is because regulators, unlike customers, do not have customers' best interests in mind.
- Most people are libertarian leaning. Drug sells do not necessarily implies forced.
- People that are hurt by stealing would prefer that resources spent on capturing drug users can be used to jail thieves instead. People that are hurt by drugs can simply choose not to do drugs.
That is at least 4 reasons why many reasonable people would support anti stealing laws instead of drug laws. Many of these are disputable. For example, some may argue that people will still use harmful drugs because they are addicted or because they do not know. However, that would at most justify total disclosure. In general, truly addictive drugs simply don't sell even if it were legal. https://silkroaddrugs.org/drug-listing-on-silk-road/
Laws are there to serve someone's interests, namely interests of those who make laws. In democratic countries, large number of people must be profited before something can become laws.
Humans are selfish. You don't have to be libertarian to see this. Prohibiting consensual act is useless. Humans are selfish. Consumers, out of their self interests, have strong incentive to pick the best product.
When fraud or force is involved, humans' self interests do not protect individuals.
For example, if we have no laws for stealing, then we would steal from each other. That's because it may be more profitable to steal than to produce stuffs productively.
It is not toward the best interest of thief victim to get their stuff stolen. However, consent is not required for stealing. A thief victim will get his stuff stolen anyway whether the victim consent or not.
Hence laws against stealing serves the interests of all potential thieves victim. Obviously, such laws are popular.
Laws against stealing actually reduce some ways a person can pursue happiness. However, if I live in a country where stealing is okay, I may get my stuff stolen too. Which is sucks. At the end, everyone is stealing from each other like in Africa and everyone is worse off. So it make sense to support laws against stealing.
Laws against stealing not only benefits potential thieves victim, but also everyone. It's the same reason why mafia have laws to kill of snitches. The laws actually benefit the whole mafia groups because now they no longer need to worry about snitches and hence will not snitch with one another.
Analysis in prisoner dilemma games shows that both parties are better off if snitches are killed.
What about laws against drug usage?
It hurts the interests of drug users.
What does the laws benefit?
Non drug users? Most non drug users can simply choose not to use drugs.
Even if I am not a drug users, I do not think it's a good idea to have laws against drug users. What about if latter someone says that breathing is bad and criminalize consuming oxygen? I'll be death.
Allowing some people to decide that drugs are not okay will set dangerous precedents that some persons will decide that other things are not okay.
Some would argue that drug usage is harm (self harm). Well, I disagree. Many people know that drug users, as consumers, have their own self interests.
When something is harmful, drug users will simply choose not to use drugs.
For example, cigarettes is legal. Cigarettes are also addictive. Then what? I simply don't smoke cigarettes. Why would I want to try something so addictive if I KNOW before hand that it would be addictive?
In fact, most people wouldn't use the harmful cigarettes if non addictive MDMA is legal. Prohibition of MDMA have made far more harmful cigarettes sells. Yes. This is arguable. However, the question ask why someone would wants to decriminalize all drugs. Because they believe that MDMA is far safer than cigarettes. That is a reasonable belief that many scientists agree.
And that is actually proven in silk road. Only soft drugs like MDMA are sold well in silk road. The hard drugs, like heroin, are are not sold.
While consumers have self interests not to harm themselves, the same cannot be said about drug regulators.
Drug regulators may get kickback from drug companies. Many more harmful drugs are on the street because safer drugs are illegal.
Death due to illegal drugs are very low at portugal where drugs are criminalized.