(For other posts in this series, see here.)
I am going to conclude this series by arguing that it was more-or-less a coincidence that led to the deep-seated animosity towards evolutionary theory in America.
The early 20th century was the time when religious people in America became alarmed that they had perhaps gone too far in separating church and state in the public schools and decided to try and reverse the trend, and this movement coincided in time with the rise in acceptance of natural selection as the mechanism evolution. This theory, with its explicit rejection of a special divine plan for the human race, became seen as a potent symbol of an anti-religious way of thinking that had to be combated. Hence it was natural to use opposition to the teaching of the theory of evolution as a vanguard action that would lead to the restoration of religious instruction in schools.