Below is a link to a very good discussion of this topic. Please follow it.
Fundamentalism in the 1920s grew out of worries on the part of "traditionalists," especially in non-urban parts of America. They saw a new culture growing up around jazz, illegal drinking, flappers, and other such things. This made them fear that their traditional values were under attack.
Many fundamentalists saw the teaching of evolution as part of this attack. Teaching evolution contradicted, in their minds, the truth of the Biblical story of Adam and Eve. Because of this, some states, like Tennessee, banned the teaching of evolution. It was in response to this ban that the ACLU got John Scopes to be part of a test case. The case was meant to publicize the law in hopes of overturning it and others like it.