Opposition to evolution has become a critical part in the growth of American fundamentalism. Fundamentalists are fighting for what they believe to be the "fundamentals" of their faith and "modernism" (the intellectual aspect of modernity) in all its forms is seen by them as an attack on that faith.
Fundamentalists are actively working to eliminate every theological, philosophical, or scientific ideas which they perceive as undermining what they believe to be orthodox, traditional Christian beliefs. Evolutionary theory tops that list because it seems to them to be playing an important role in degeneration of a society that has abandoned true Christianity. All of this has only intensified in the wake World War I, leading to laws like the Butler Act.