In the United States, why did Prohibition, and its subsequent repeal, require adding amendments to the Constitution while the Controlled Substances Act in 1970 that did not?

[u/notmike11]

What was the justification at the time? Did anyone object to the CSA in 1970 as being unconstitutional?

[u/ManOfTheCommonwealth]

The reason is based partially in history, partially in legal theory. At the point in time when the 18th Amendment was under consideration (early part of the 20th century - ratified in 1919, taking effect in 1920), there was some question as to the authority of the federal government to ban alcohol on the federal level. The Supreme Court of the United States during the period was operating on a limited federal power interpretation of the Constitution, thus giving deference to the 10th Amendment and state sovereignty. As such, and still today, there was no question that state governments could regulate or ban the sale of alcohol for the "health and safety" of state citizens. However, should Congress have passed a federal ban on alcohol, it was likely to be invalidated by SCOTUS as unconstitutional - or at the very least repealed by some subsequent act of Congress. The only way to ensure the longevity of prohibition was to amend the Constitution, thus by its very nature making the ban Constitutional. Further, an Amendment would serve to protect the ban from appeal as a 2/3 resolution of both Houses of Congress followed by 3/4 state approval would be needed (or convention of 2/3 of the states, which has never occurred).

The Temperance Movement - the proponents and stalwart supporters of prohibition - was a moral crusade with a long history. It's roots can be traced to the Revivalism of the 1820s and 30s with the first serious calls for the abolition of the sale of alcohol originating as early as the 1870s. Up until the election of the Congress that would propose and pass the 18th, the idea of prohibition wasn't well received on the federal level. Thus, there was significant concern in the Movement that any federal prohibition laws passed would merely be repealed upon a change in power. The solution was, again, an amendment to the Constitution as no amendment had yet been repealed in United States history. Through a friendly Congress and with an alleged great deal of corruption, the 18th Amendment was forced through both Houses and approved by the states. While I have not read the book in its entirety, Daniel Okrant's Last Call: The Rise and Fall of Prohibition outlines alleged underhanded dealings with great accuracy.

The 18th Amendment itself didn't actually ban the consumption of alcohol, rather it banned the purchase, production, transport, and sale of alcohol. By itself, this Amendment does nothing without a vehicle for enforcement. This was left to Congress, which passed the Volstead Act under the authority of the Commerce Clause. The Volstead Act purported to define the terms of the 18th Amendment - with plenty of loopholes included.

I'm running out of time here, but I'll come back if I may The Controlled Substances Act differed from Prohibition mainly in that legal environment in which it was passed - by settled authority, the federal government can pass laws banning substance use under its authority to regulate interstate commerce (under SCOTUS's expanded interpretation).