Prohibition (and in fact, any prohibition) is unsustainable because of all the bad it caused and because it simply did not work and in fact made things worse. Organized crime was taking over the cities, and people were still drinking, and what they were drinking was dangerous because there was no regulation of it since it had been banned. There were poisons in the bootleg booze, and hundreds upon hundreds died as a result. Meanwhile, the rich bought imported booze and kept massive stockpiles, which the working man was the one that got screwed over. It was just broken, and ignorant, and misguided. It failed, and it failed miserably.
And come the Depression, no one cares about the moral finger wagging that was Prohibition, and people figured it wasn't working, and the nation needed money, so repeal Prohibition and tax liquor.