It is because fundamentalist support was strongest in the 1920s.
Fundamentalism is usually characterized by scholars as a religious response to modernism, especially the theory of evolution as an explanation of human origins and the idea that solutions to problems can be found without regard to traditional religious values.
Generic fundamentalism refers to a global religious impulse, particularly evident in the twentieth century, that seeks to recover and publicly institutionalize aspects of the past that modern life has obscured.
Historic Fundamentalism shared all of the assumptions of generic fundamentalism but also reflected several concerns particular to the religious setting of the United States at the turn of the century.
Read more about The Rise of Fundamentalism at National Humanities Center.