Naturalism excludes an absolute Creator and asserts that life somehow emerged out of matter and energy by itself and evolved into its present forms. In order to be persuasive and credible, each view needs a consistent explanation of how things got here. Evolution is therefore essential to naturalism in the same way creation is to theism. For thousands of educators then, evolution provides a thorough-going explanation of reality. The connection is that this naturalistic explanation of reality has certain companion ideas about morality.
Theism postulates that reality includes a moral order, but naturalism excludes a lawgiver.
Naturalistic evolution and moral relativism are like two rooms in the same house. Hence young minds influenced with modern naturalism conclude that moral values are whatever we happen to declare them to be at any given time. Modern biology has undermined this assumption. It is doubtful that naturalism could have gotten anywhere in the school systems had it been introduced through its position on morality. But it found a back-door acceptance by its association with evolution, especially since evolution is taught almost exclusively.