By the 20th Century, "Historical Fiction" had fallen into disrepute. Many people associated it with "bodice rippers" and other forms of trashy romance. It was -- and remains -- "genre fiction," and not very respected genre fiction at that.
Of course, that was largely because for all the good historical novels that were still being written, the market was flooded with far too many "bodice rippers" and "costume dramas," in which characters with modern-mentalities dressed up in fanciful costumes to prance around in worlds littered with anachronisms. For a reader unfamiliar with a particular period of history, it was difficult to distinguish between well-researched and sloppily researched novels. As a historian, I have too often picked up a book praised by reviewers for being "meticulously researched" only to encounter glaring and offensive (to a historian) errors on nearly every page. There is a great deal of trash out there masquerading as "historical" fiction!
In the 21st Century there appears to be some gentle course correction in progress, with many serious readers recognizing that -- despite all that fake "historical" stuff -- real historical fiction based on comprehensive research and fashioned by master storytellers is a legitimate "genre." As a historian who also writes novels, I am pleased by this trend. After all, historical fiction is arguably the oldest of all the genres.
What, after all, was the Iliad if not historical fiction? It was developed hundreds of years after the events it describes and intended to make those events comprehensible to contemporary audiences. Admittedly, for years it was dismissed as "pure fiction" but ever since Heinrich Schliemann uncovered the archaeological remains of the "fabled towers of Troy" -- and subsequent archaeologists have established that it was sacked and burnt etc. it is clear that the Iliad was not "pure" fiction but rather historical fiction. Thus the modern, Western tradition of literature starts with a (magnificent!) work of historical fiction.
Other examples of literary masterpieces that fall into the "genre" of historical fiction are, of course, Shakespeare's Histories (including great works like Hamlet and Henry V), War and Peace by Tolstoy, and Margret Mitchell's Gone with the Wind. Admittedly, Shakespeare wasn't a great historian, but the quality of his writing makes up for that deficiency! Tolstoy and Mitchell, on the other hand, were meticulous in their research, albeit many today will find Mitchell's point of view politically incorrect.
Modern historical novelists ought to keep in mind that unlike our colleagues writing vampire stories, steamboat punk and even science fiction, which are all relatively new inventions, we are following in the (very large) footprints of some truly great authors. We have a tradition to maintain -- and if we do it right, recognition will (hopefully) follow.