It hasn’t been. Books pass into the public domain 50 years after the death of the writer (possibly a longer period in the US but I have this vague idea that they tidied up the laws on an international basis a decade or so ago) So for a significant period of time after the writer’s death, the writer’s heirs get to receive royalties and income from the works.
A good example of this is(are?) H.P. Lovecraft’s works, which are public domain, as opposed to those of J.R.R. Tolkien’s works. Sr. Tolkien has heirs who will valiantly(violently?) oppose anyone’s effort to use his world/characters/what have you.
Lovecraft, OTOH, doesn’t appear to have anyone who feels nearly so vehemently about the issue - regardless of whether he has any living descendants.
So, if I want to write a Lovecraftian story there will be no reprecusions; if I were to write a story using the world of The Lord of the Rings and publish it (anywhere) and the Tolkien heirs were to find out about it the best I could hope for is a Cease & Desist, if not an outright lawsuit.
When did Lovecraft die? If his heirs are still receiving royalties, then the publisher will defend the copyright, rather than the estate. Tolkien hasn’t been dead 50 years, so Allen & Unwin can and do defend the copyright. Generally a writer doesn’t defend the copyright, there is a clause in the contract where the writer states they are the originator of the work and they own the ideas (paraphrasing and too lazy to go and actually find a contract) and then the publisher says they will defend copyright. The writer indemnifies the publisher against being sued if the writer has in fact stolen the work, or the ideas therein.
Generally if the work remains in print, then the publisher is still making money so they will take people who infringe on the copyright to court. If the publisher declines to defend the copyright, then the writer can choose to. Usually the writer (or the writer’s literary executor if the writer is dead) won’t because it costs too much.
My WAG is that Lovecraft has passed into the public domain because of the passage of time while Tolkien hasn’t. Once the fifty years are up (I am not certain of that period of time, i do know it is a significant period though.) then the work passes into the public domain but not before.
If a book however is out of print for a year (that period of time can vary depending on the original contract) and the publisher has no intentions of a reprint, then the writer or the literary executor can request the reversion of rights and then they can assign those rights to whoever they please. It is possible but unlikely that something like that has happened with Lovecraft.
Lovecraft died in 1937, to be precise. In 1939, his protege August Derleth, and Donald Wandrei estabished Arkham House publishers who for the last 61 years have been responsible for maintaining Lovecraft’s creations while at the same time encouraging new authors to make use of the settings, terms, and “feel” that Lovecraft created. (Which was always what Lovecraft intended).
Robert Bloch, who was the model for “Robert Blake” in “The Lurker in the Darkness”, has written (IMHO) some of the best Lovecraftian fiction.
The earliest non-Lovecraft Mythos work that I’ve read was Frank Belknap Long’s “The Hounds of Tindalos” published in 1929. (While H.P. was still kicking around.)
Stephen King wrote “Jerusalem’s Lot” in his Night Shift collection, a piece that was very very Lovecraftian, including references to numerous other Lovecraft stories. His short story, “Night Shift” is basically just a slightly reworked version of Lovecraft’s “The Rats in the Walls”.