Murray Rothbard gives the history.
The libertarian movement was once a mighty movement, hardcore but not kooky, part of the mainstream of American ideological and political life. In the 18th and 19th centuries (for example, in the Jeffersonian and Jacksonian movements), libertarians were even the dominant political force in the country. America was, indeed, conceived in liberty. But right now, I’m not going back that far: I’m talking about the origins of the modern 20th century movement. For various reasons, the Progressive movement had wiped out 19th century intellectual and political libertarianism, and, by the 1920s, it was reduced to a few vibrant but lone intellectuals such as H.L. Mencken and his friend, Albert Jay Nock.