Why is evolution a hard topic to cover?

Evolution is a difficult concept to cover because it is contrary to popular religious teachings.
Evolution itself is a rather simple thing to teach. All living things have a common ancestor. Adaptation occurs over time as natural pressures make certain genes advantageous; this is natural selection. Speciation is when a line from one species has adapted to the point of being different from its parent species. Teaching evolution itself, even in depth, can be done very efficiently.
People might resist evolution because they believe a real divine power makes more sense. For example, in the American South, many fundamental Christians believe in Genesis theory, where God created the world over seven days, the Earth is only 6000 years old, and God once sent a massive flood to destroy the world. These Christians will lobby their state and city governments to ensure that what they believe is taught instead of or alongside what they think is a lie. Because they have lobbying power in these places, these individuals can be very successful.
Most schools are nervous about teaching evolution, as it can be a touchy subject. This means that they will either simply not cover it or present a religious view on the subject at the same time, implying both are credible theories.