Why was James I especially interested in witchcraft?
Concerning James I and witchcraft, I can tell you what textbooks used to teach Brit. Lit. say about the subject. In other words, I am not an expert on this and haven't done research on it, and some other editor that has, and certainly university professors that specialize in Shakespeare, could tell you more. But its been a couple of hours and no one else has answered so I'll try to help you out.
Witchcraft, of course, is still an issue in Shakespeare's time. The English renaissance and the moveable type printing press and advancements in science, etc., did not eliminate superstition: at least not completely and not for everyone. James I had written against witchcraft, equating it to demonology, and telling how God fights against it by revealing the witches for what they are, and aiding in their capture, torture, confessions, etc.
Reportedly, James I was then confronted by a woman claiming to be a witch, who told him things about his life that no other human should have known.
Outbreaks in witchcraft occurred in Scotland when James I was James VI of Scotland, before he became James I of England. This may be what drew his interest.
Presumably, the ruling monarch's interest in witchcraft contributed to Shakespeare's decision to use witches in Macbeth.