1. Author publishes a dozen novels over the first 20 years of his/her career. All of them are good, but none of them sell well.
2. Author does not pay the copyright renewal fees because they are too expensive. Anyone can now copy those books and pay nothing.
3. Author's next book is well-received. The author is hailed as a genius and there is suddenly a ton of interest in the author's earlier works.
4. Everyone but the author makes money on the author's older works.
Edit: The point here is that when a work is successful, which is rare, it is unfair for everyone other than the author, who is most responsible for the work, to profit from it.