Well you can't really pinpoint to one single reason but if you'd want to make it easy the answers would most likely come to racism and Harry Anslinger.

The first regulations of Cannabis occurred with the second Opium Convention in 1925 when Cannabis trade due to pressure from various sides (mainly Egypt though) was set to be regulated like cocaine and heroine. The Opium conventions were started to regulate (as the name implies) the opium problems that had been rampant during the late 19th and early 20th century but since Egypt was an important trade route to get opium to Europe they weren't really too happy with opium being prohibited/strongly regulated. At the second Opium Convention they then put pressure on the other states (threatening the German Empire with a import embargo on cocaine and heroin which at that time were produced by German pharma companies, Merck and Bayer respectively).

So, this was the global situation, coming to the US now we have to see that after prohibition of alcohol failed tremendously and was lifted in 1933 there were still puritanical movements in the US and they needed a new scapegoat drug to be appeased. Also, a lot of people who worked to keep prohibition running were facing unemployment so the US government founded the Federal Bureau of Narcotics (predecessor of the DEA) and Harry J. Anslinger was appointed as the first commissioner of the agency. Anslinger was eager on promoting the prosecution of cannabis users because he felt that would give the agency more significance and had the side-effect of mainly affecting young, male Blacks and Hispanics who during that time were the "main" cannabis consumers and Anslinger himself was a pretty big racist, so he could kill two birds with one stone.

Anslinger started an incredible anti-cannabis propaganda movement depicting it as a drug that makes you violent, insane, murderous, criminal and making white women have relations with unfavorable people (non-whites).

To quote Anslinger:

There are 100,000 total marijuana smokers in the US, and most are Negroes, Hispanics, Filipinos and entertainers. Their Satanic music, jazz and swing, result from marijuana usage. This marijuana causes white women to seek sexual relations with Negroes, entertainers and any others. It is a drug that causes insanity, criminality, and death -- the most violence-causing drug in the history of mankind.

Along with his fear-mongering speeches there were billboards put up, radio commercials etc. What Anslinger also did was separating hemp from marijuana. He adopted the name marijuana (as the drug was called by Hispanics who used it) to describe the "killer drug" cannabis while hemp was still a steady fixture in US economy and its production was promoted (especially once the US joined WWII). People lost the connection that the two names are basically describing the very same thing but it was an interesting scheme to avoid inconvenient questions.

In 1937, Anslinger testified in congress that marijuana is even worse than opium and told some made up stories about poor white women who were abducted by "Negroes" and when they returned were pregnant or had syphilis. Or about a worker who smoked a marijuana cigarette and turned to raping his 6-year old son...you get the picture. So, the Marihuana Tax Act went into effect. This is usually where it comes up that the law was only instated because of DuPont and William Randolph Hearst who feared their paper market would slip into the hemp sector, however the actual evidence for that isn't that strong (mainly because compared to wood hemp isn't really as great for paper production as some might think).

Also we shouldn't forget that until 1937 there have been at least 3-4 years of extraordinary anti-cannabis propaganda and that surely left an impact on society.

Not everyone though, because in 1938 the work on the La Guardia report (New York Acedemy of Medicine commission appointed by NY mayor La Guardia) started and the commission published their findings in 1944 and basically contradicted the whole propaganda Anslinger had spewed. They found out (amongst other things) that marijuana did not form an addiction (in the medical sense), was not popular amongst children/teenagers and also did not lead to using stronger drugs like opium or cocaine, and that generally the whole concern about the "catastrophic effects" of marijuana is pretty much unfounded.

Anslinger reacted by calling the report unscientific and prohibiting the doctors who worked on it to do any more research on cannabis and interfering in any research that would lead to an outcome unfavorable to the government's/Anslinger's point of view.

Later Anslinger moved to the newly founded UN Drug Commission and spread the same propaganda but now on a global level leading to the UN's Single Convention on Narcotic Drugs making cannabis a schedule I drug with the same harm/effects as heroin, morphine and cocaine but this might lead to far now.

Source: Did a presentation on it 2 months ago, my main sources are mainly German though (that's why I only linked Wikipedia articles). This is my first time writing an answer here so I hope it's okay as it is otherwise I'd be happy to dig out some English sources if the mods wish.

Edit: Some further sources,

Die Anti-Hanf Kampagnen gives a nice historical overview on Anslinger's work and the steps leading to cannabis prohibition.

Harry Anslinger - Kampf gegen Cannabis

Wann und Warum wurde Cannabis verboten?

Why is Marijuana Illegal?

Some further insight on the Marihuana Tax Act

Granted, most of those are second hand sources but they all provide more literature for further reading and since my main focus was on legalization approaches I did not invest into more primary literature on the prohibition history.

Also did some spelling and grammar editing and edited the original text of the second Opium Convention and the Marihuana Tax Act into the text.

I've read that the cotton industry felt threatened by hemp and pressured lawmakers - is there any truth to that?