Why does Christopher dislike Sir Arthur Conan Doyle?

From the text:

I like Sherlock Holmes, but I do not like Sir Arthur Conan Doyle, who was the author of the Sherlock Holmes stories. That is because he wasn’t like Sherlock Holmes and he believed in the supernatural. And when he got old he joined the Spiritualist Society, which meant that he believed you could communicate with the dead. This was because his son died of influenza during the First World War and he still wanted to talk to him.