The 1920s are typically seen as a time when urban areas in the United States were becoming more modernized while rural areas remained traditional. This was the cause of a great deal of conflict between the two regions and, more generally, between modernizers and people who wanted to retain traditional ways. The Scopes Monkey Trial is one of the greatest examples of this conflict.
In the 1920s, urban America was changing. Huge numbers of immigrants had flooded to American cities, making them demographically different. Technology was also changing the cities. City dwellers had things like electricity and running water and radios and movie theaters. These were things that set them apart from people in rural areas and which made their lives very different. As life changed, so did values. Urban America was a place of jazz and flappers. It was a place that was moving away from the traditional values that still dominated the countryside.
The Scopes Trial shows us the differences in values and attitudes. It took place in Tennessee, which was a rural state. It took place in a very rural part of the state. The Tennessee law that banned the teaching of evolution was a law that was meant to uphold traditional religious ideas against the ideas of modern science. Modernized Americans believed in science and did not believe in the literal truth of the creation story. However, more traditional, largely rural Americans still did believe in the literal truth of the Bible. This law was one aspect of their attempt to maintain the supremacy of their beliefs.
The Scopes Trial itself also played up the differences between rural and urban America. Urban newspaper and radio reporters flocked to Dayton to cover the trial. Many of them (most particularly H.L. Mencken) portrayed the prosecution and the people of Dayton as backwards and ridiculous. The media clearly sided with the more modern ideas and thereby helped to propagate the idea that there was a fundamental difference between urban and rural America.
The Scopes Trial came about because America was modernizing and some people did not want that to continue. It came about because modern, urban America was moving away from traditional religious beliefs while traditional, rural America continued to hold to those beliefs.