There were two parts to the Prohibition act: The Volstead Act and a Constitutional ban, the 18th. Amendment. The Volstead Act passed in 1919 enabled the US government to enforce the 18th. Amendment.
The 18th Amendment was introduced into the Senate in 1917, and it was successfully ratified by 1919, when the need for the Volstead Act to enable its enforcement became clear. Under the 18th Amendment, intoxicating liquor was essentially prohibited within the United States. The law was passed in response to the temperance movement, which had gathered large numbers of followers. Adherents to the movement believed that the consumption of alcohol was harmful, and that society in general would benefit if alcohol was banned.
So basically it was for the "good of the nation" and I'll bet that sometime down the road they'll do the same for tobacco; you know, it's for the children.
I wonder if anyone has realized that the large numbers of new immigrants the left is importing are from places where the majority are smokers.