Although I cannot get into the larger social backdrop against which these laws and regulations were promulgated, I can shed some light on the legal history and legal reasons why Prohibition was accomplished through a constitutional amendment (and later prohibitions against drugs were not).

To begin, it's important to discuss a few generally-applicable legal concepts.

In general, state (and other more local, such as county or city) legislatures have what is called a general “police power,” which refers to the ability of a government to pass any law concerning any subject that it believes is necessary to benefit its constituents. Congress, however, only has the authority to pass laws pursuant to one of the “enumerated powers” listed in the Constitution. As such, when Congress passes a law, it must be an exercise of one of Congress’s enumerated powers to be constitutional (i.e., the law must relate to one of the specifically-articulated subjects that Congress is permitted to regulate, as listed in the Constitution).

For example, since one of Congress’s enumerated powers is the power to print money and regulate the value thereof, Congress is permitted to pass laws creating the U.S. Mint and giving the U.S. Mint the authority to print money of certain denominations. In contrast, because the Constitution does not give Congress the enumerated power to, for instance, outlaw gender-motivated violence against women, Congress cannot pass a law that allows women to sue perpetrators of violence against them in federal court (See, e.g., United States v. Morrison, 529 U.S. 598 (2000)). (This does not mean that violence against women is totally legal, however; it just means that Congress cannot pass a federal law on the subject. State legislatures can pass, and have passed, laws outlawing violence against women, pursuant to their general police power).

At the same time, however, the United States Supreme Court has held that the enumerated powers listed in the Constitution also give rise to certain “implied powers,” based on the Constitution’s so-called “Elastic Clause,” which gives Congress the power to enact all laws that are “necessary and proper” to carry out its enumerated powers. For example, because the Constitution gives Congress the enumerated power to “establish” the postal service, Congress also has the implied power to pass a law that prohibits sending bombs through the mail, even though prohibiting the mailing of bombs is not, strictly speaking, a law that relates to “establish[ing]” the postal service. This principle was first articulated by the United States Supreme Court in the 1819 case of McCulloch v. Maryland, wherein the Court held that Congress had the implied power to establish a national bank, since creating a national bank was a necessary and proper exercise of Congress’s enumerated power to imposes taxes and spend tax revenue to promote the “general welfare of the United States.”

Another one of Congress’s enumerated powers is set forth in what is known “Commerce Clause,” which gives Congress the power to regulate what is known as “interstate commerce.” In short, interstate commerce is commerce that involves multiple states, whereas “intrastate commerce” is commerce that occurs entirely within one state.

At the time the 18th Amendment and the Volstead Act were passed—i.e., when Prohibition started—the conception of what constituted “interstate commerce” was significantly more narrow that it would later become. Under that more narrow idea of what constituted “interstate commerce,” Congress only had the power to regulate alcohol if it was sold and/or distributed across state lines; Congress would have no power to regulate the manufacture or sale of alcohol that was produced and sold entirely within a particular state (since that would be intrastate commerce).

Put another way, although Congress could, pursuant to the Commerce Clause, ban the manufacture and sale of alcohol in interstate commerce, only individual states could ban the manufacture and sale of alcohol that occurred entirely within one state. And, while some states may have happily passed such a law, other states would not have done so. As a result, the only way to effectuate a nationwide ban on the manufacture and sale of alcohol was to pass a constitutional amendment that would expand Congress’s power to regulate alcohol regardless of whether it was part of interstate commerce. This is why, at the time of Prohibition, the 18th Amendment was necessary.

During the New Deal era, however—which is when the federal government began to really get involved in regulation of a number of things, such as illicit drugs—the definition of “interstate commerce” began to expand. The literal textbook example of this expansion comes from the case Wickard v. Filburn, which was decided in 1942.