It seems like such a narrow and arbitrary thing to become a constitutional amendment instead of a standard law, why was it put in the constitution?
This involves the idea of the general police power.
In 1887, in the case of Mugler v. Kansas, the Supreme Court had held that Kansas could prohibit the manufacture and sale to the general public of all alcoholic products in the State of Kansas under the general police power. It is usually understood that the general police power over individuals is a power reserved to the states under the Tenth Amendment.
Accordingly, the Eighteenth Amendment was required in order make the Volstead Act constitutional.
Subsequently, the Supreme Court discovered the previously unsuspected wonders of the commerce clause, which has obliterated the Tenth Amendment.
Clearly, if the Supreme Court had been as imaginative or as unprincipled (take your pick) in 1918 as it became in 1942, the Eighteenth Amendment would not have been needed to uphold the Volstead Act.