Why was the prohibition of alcohol in the US written onto the Constitution itself instead of a piece of federal law? What were the reasoning behind it, and were there any debates/criticisms at the time regarding the appropriateness of including the prohibition of alcohol in the Constitution? In my mind, for example, the Constitution should reserved for "more important" legislations such as fundamental rights of the citizens.
You may find /u/manofthecommonwealth 's response to this thread will largely answer your question.
In short, legal theory at the time took a much more limited view toward federal power. At the time they believed the Feds couldn't regulate alcohol barring an authorizing passage in the constitution. Prior to the 18th amendment states could and did individually prohibit alcohol. But it was viewed as one of the things that was reserved to the states rather than the federal government. Thus an amendment was necessary.
In the decades since Prohibition, federal power and authority has expanded tremendously. It's likely that if prohibition were to become a national policy today, the Feds wouldn't bother with an amendment, they'd just declare it legitimate via the commerce clause, much in the way they do with other recreational drugs.