Because there are correct answers and wrong answers, and a person's faith has little to do with the reality of right and wrong. Our society has become a little schizophrenic, thinking that right answers exist for any area of life except religion and morality. Again, if God exists and spoke, then absolutes exist and we have a basis for deciding right from wrong. Yet, people reject Christianity, not because they researched the historical evidence and found it lacking, but because they simply don't like what it says. That's a classic bias.
Many people believe that the world was created rather than evolved.
But since those theories imply a god, we cannot discuss the merits of these theories in the classroom because the state cannot be a promoter of religion. The people should not be insulated from open, intellectual discussion of God and the Bible, especially if it might be all true. It's a favorite war slogan to toss out to avoid facing the music. The religion department shook for days. He was asked to leave the university and join a Christian college because he was judged guilty of promoting a religion.