It might be to do with the remaining durations of some kinds of UK (and US) copyrights counting down from the death of the author. (I don't know whether those r

It might be to do with the remaining durations of some kinds of UK (and US) copyrights counting down from the death of the author. (I don't know whether those rules were in place during Conan Doyle's life, but his copyrights might have been retroactively extended.) See https://en.m.wikipedia.org/wiki/List_of_countries%27_copyright_lengths https://en.m.wikipedia.org/wiki/List_of_countries%27_copyrig...