Because fuck public domain, that's why.
Earlier this year US courts firmly decided that all the Sherlock Holmes books published before 1923 (the current limit for copyright) are in the public domain, and thus the estate has no right to claim licence fees on any works based on those books (tv shows, movies, critical books, fanfiction and everything else you can think of). That lawsuit started when Leslie Klinger - an author of an upcoming book about how Sherlock Holmes influenced other stories (basically a book of literary analysis, and I was under the impression those are usually fair use anyway) - was being pressured into acquiring a licence filed for declaratory judgement.
So now the estate is asking the US Supreme court to take on the suit for two reasons:
1. Because apparently you can't determine whether a work was infringing if it's not finished yet, and Klinger hasn't finished his book yet. (But that fact won't stop you from pressuring the publisher for licence fees apparently.)
2. Sherlock Holmes is a "developing character" and because one more book of his adventures is still under copyright the whole character should be under copyright. Which is obviously ridiculous (AFAIK you can't really claim copyright of characters independent from original stories/set medium) and a defense they've tried before (and failed).
There's a much better write-up of this shenanigans at Techdirt. It's not very likely that the Supreme court will take this case, but who knows.