Explain Like I'm Five

Because drugs are a convenient pretext to harass minorities,

“You want to know what this was really all about? The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar left and black people. You understand what I’m saying? We knew we couldn’t make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did.”

-- John Ehrlichman, White House Counsel and Assistant to the President for Domestic Affairs under Richard Nixon

and heavy-handed anti-drug legislation remains a useful pretext to the government to this day for largely the same reason, as well as driving modern slavery (gotta keep those numbers up in for-profit prisons).

The racism part mentioned by the others is certainly a large portion

However there's also lobbying from both Tobacco and Alcohol groups against weed. There's a fear that since weed is less addictive than both that people will use it as an alternative and that means less money from addicts. So they help push anti-weed rhetoric to keep it illegal and they have a lot of money to do so

In short, racism.

Marijuana was largely banned because it was associated with Mexican immigrants and African Americans. It was claimed the drug led to violence, insanity, and race mixing.

Even today, black people are about four times as likely as white people to be arrested on marijuana-related charges even though they use the drug at similar rates to white people.

Private prison industry, racism, and religious people

There’s billions of dollars spent on the “war on drugs” and cannabis is an easy target. Plus, there’s decades of messaging about the evils of cannabis that makes a lot of people nervous about it.

Because it would unburden institutions, let private retailers grow it saving the tax payer money, and people would no longer shell out for insurance. No body wants to help you, are you stupid?

I wish weed was illegal here in Germany so I could open my windows and air the place out. Especially fun when it's hot out during the day and it's not possible to let in the cooler air at night because the twat downstairs lights up at 5 am.