Why was a constitutional amendment required to out

Why was a constitutional amendment required to outlaw the production, transport and sale of intoxicating liquors (alcoholic beverages) in the U.S. but not in case of cannabis prohibition, which was achieved by only a simple act of Congress? 

Answer:  The hypocrisy is explained in that Congress gradually usurped power from the states as the arbitrers of cannabis beginning in 1937 (under FDR) with a steep federal tax on cannabis. The process was completed by 1970 with the passage of the Controlled Substances Act of 1970 which classified marijuana classified as a Schedule I drug. A return of power to states in regard to cannabis is what we are seeing now.

It's no random coincidence that, with the end of alcohol prohibition in 1933, there came increasing pressure on cannabis to be taxed, regulated and eventually outlawed. 

In the early 1900's, the judiciary accepted that the federal government did not have the constitutional power to outlaw alcohol or drugs, in keeping with the 10th amendment. Only local governments and the states had that power. That is why alcohol prohibition on a federal level required a constitutional amendment, ratified in 1919. It could not be done by merely an act of Congress. The judiciary of the time recognized that the 10th amendment blocked Congress from what were considered local and state affairs and that direct regulation of medical practice by Congress was not empowered by the commerce clause.

Amendment XVIII (Ratified on January 16, 1919 by all 48 states except Rhode Island and Connecticut. Amendment took effect on January 17, 1920.)

Section 1. After one year from the ratification of this article the manufacture, sale, or transportation of intoxicating liquors within, the importation thereof into, or the exportation thereof from the United States and all the territory subject to the jurisdiction thereof for beverage purposes is hereby prohibited.

Section 2. The Congress and the several States shall have concurrent power to enforce this article by appropriate legislation.

Section 3. This article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by the legislatures of the several States, as provided in the Constitution, within seven years from the date of the submission hereof to the States by the Congress.

The 21st Amendment repealed the 18th Amendment on December 5, 1933.