One very good reason can be seen today: after some of the books from the Sherlock Holmes series went into public domain, artists today are free to use and create derivative works based on/off of them.
Case in point: "Elementary" [1] is a Sherlock Holmes series where both Moriarty and Watson are women, Sherlock Holmes lives in NY and he doesn't get along with Inspector Lestrade.
If things never fall into public domain, doing something like this can be either very expensive (e.g. paying royalties) or plain impossible (e.g. the author simply doesn't allow it).
Copyright laws were meant to incentivize authors to create more, but no amount of incentives will make Sir Arthur Conan Doyle create more interesting stories, about Sherlock Holmes or otherwise.