Then why did they go to the trouble of amending the constitution?
"An amendment to the Constitution obviously appealed to temperance reformers more than a federal statute banning liquor. A simple congressional majority could adopt a statute but, with the shift of a relatively few votes, could likewise topple one. Drys feared that an ordinary law would be in constant danger of being overturned owing to pressure from liquor industry interests or the growing population of liquor-using immigrants. A constitutional amendment, on the other hand, though more difficult to achieve, would be impervious to change. Their reform would not only have been adopted, the Anti-Saloon League reasoned, but would be protected from future human weakness and backsliding."
Furthermore, the 21st amendment no only repealed the 18th (Section 1), but removed the power to regulate alcohol from the federal government and returned that power exclusively to the states (Section 2).