Because if people are curious about things other then Christianity, that would be awful.
“But even if Harry Potter’s magic isn’t of the occult, it still carries with it serious dangers. First, Rowling’s stories—unlike Lewis’ or Tolkien’s—are neither a Christian allegory, nor do they subscribe to a consistent Christian worldview. And second, we live in a culture that glorifies and promotes witchcraft and the occult. No matter what the essence of Harry’s magic, the effect of it is undoubtedly to raise curiosity about magic and wizardry. And any curiosity raised on this front presents a danger that the world will satisfy it with falsehood before the church or the family can satisfy it with truth”
- Focus on the Family