Actually it is understandable if you understood the previous world view of man place in the world.
So though you are correct that belief in biological evolution is not incompatible with religious belief it is the principle of common descent that is the aspect of Evolutionary theory that bothers religious conservatives and understandably so.
For millennia humans held themselves to be above animals. The prevailing notion that, we are not animals, we are civilized. As a social distinction between humans and other animals which makes humans distinct from other animals.
Then evolutionary theory comes along and turns this belief on it's heads...ok...we may be civilized...but we're still animals. This is not a particularly palatable notion to many people not steeped in the tradition of science but who have strong religious beliefs.
So it's really the fact of common descent that religious conservatives find offensive about evolutionary theory.