While by definition the prohibition era of the 1920s was not unconstitutional, could it be argued that due to its nature it was viewed as infringing upon the public's civil liberty and as a result was all but completely disregarded by U.S citizens.
It failed due to a variety of factors. The surge in crime that prohibition brought coupled with the sheer inability of any government to enforce such a law rendered it largely idealistic. This would undercut support for the 18th amendment as time went on. Additionally when it was passed drinking was associated with German Americans. The first world war propaganda equated being german to being anti-american. This faded as time went on, thus popular support was further eroded. Ultimately prohibition failed because it was an utopian project, and because it was unenforceable. Several books i would tell you to read would be "Prohibition Is Here to Stay” The Reverend Edward S. Shumaker and the Dry Crusade in America by Jason S. Lantzer, and Amendments XVIII and XXI : prohibition and repeal edited by Sylvia Engdahl