"While prohibition never worked with alcohol.."
This is incorrect. During prohibition most people followed the law and did not drink alcohol. We know this because the death rate for alcohol poisoning sky rocketed when the ban was lifted. Did some people not follow the law? Of course. That is true of every law, though.
Why was alcohol illegal?
To protect women and children. It was a movement among women who didn't have control over family finances to force their husbands to be better husbands and fathers. Alcohol costs a lot of money out of a family budget and causes domestic violence.
Why was it made legal?
Because making it illegal caused serious crime. Making the mob very wealthy is more detrimental to society than having some bad fathers who drink too much. Also, alcohol was popular. Most people like to have a drink from time to time and they aren't abusing their families. This is a democracy, so if we like to do something, and its constitutional, it should be legal.
Why are drugs illegal?
Same reasons. To protect other people from the actions of addicts. When someone becomes addicted, they don't take care of their families because they can't. People who are high all the time may not make sure their child has their homework finished or go to the grocery store as often or keep the house as clean. Rarely, they may even commit crimes against strangers, but mostly they are just bad friends and parents. There is no law against being a crummy friend or family member, but it still hurts other people. Addiction is very hard to treat, as well. So making the drugs illegal prevents people from even experimenting.
Why would legalizing it be better than legalizing theft or murder?
The laws against murder and probably grand theft to a large degree, don't prevent these actions. They provide punishment to offenders so that those that are harmed will get justice. If these actions were not illegal, the victims or there families would seek justice outside the justice system and more people would be hurt. People don't feel the need to violently suppress consistent pot use, since as I said before the main effect is crummy personal relationships, so this is a false comparison.
Why would legalizing it be better than legalizing drunk driving?
The laws against drunk driving are a form of regulation of drug use. The intention of the law is to allow alcohol to be legal, while mitigating its most negative effects. Most proponents of drug legalization, although not all and its difficult to generalize, think some regulation is beneficial. For example, laws that prohibit sale to children or laws regulating purity of the substances being sold, as is already the case with alcohol. Again, I think this asks the wrong question.
Why would we want to make drugs legal, especially pot?
First, because they can be regulated and that works. People will follow a drug regulation law, such as one against drunk driving, so there is no need for it to be completely illegal. With regulation most of the harmful effects are mitigated and people can enjoy their favorite drug.
Two, as with alcohol, making it illegal caused serious crime and made serious criminals very rich. Pot is much less harmful than alcohol, but making it illegal caused just as much serious crime.
Three, pot is a popular drug. Most people try pot sometime in their life, even if they aren't consistent users. This means most people are committing a felony. It's simply bad jurisprudence to make a common offense a felony.
Four, crummy parents aren't made better parents by being in prison. Many children in the US grow up without one or both parents because they are in prison for a pot conviction. Having no parents is far worse than having ones that are relatively not ideal. Therefore the law does not actually improve the lives of children and is counterproductive in its intent.
There are other laws against "victimless crimes," such as prostitution, but these atleast don't harm the group they are intended to help (women), gambling (families), truancy and trespassing (property owners), ect. I think this is the main difference.