why was a constitutional amendment necessary to prohibit alcohol?

Because the prohibitionists could, and they had popular will. They'd already done much with statuatory law. (I guess there're a bunch of current groups that would love to put their agenda in the constitution, too.) From the National Archives:

" Temperance societies and Progressives alike saw the need for more governmental control and involvement in citizens' lives. They were successful in passing several laws at the local level. Between 1905 and 1917, states across the nation were imposing laws that prohibited the manufacture and sale of intoxicating beverages. They did not stop there, however. The temperance societies began to push to change American society and elevate morality through national legislation. In 1917, the House of Representatives wanted to make Prohibition the 18th Amendment to the Constitution. Congress sent the amendment to the states for ratification, where it needed three-fourths approval. The amendment stipulated a time limit of seven years for the states to pass this amendment. In just 13 months enough states said yes to the amendment that would prohibit the manufacture, sale, and transportation of alcoholic liquors. "

-jk