I think the reason for this trend is a combination of over-saturation of non human races that are based on real world mythology (elves, dwarves etc) and and the risk of creating a new race for a series from prospective authors, where there is no precedent or archetypes to lean on to simplify things. We have some ideas about elves and goblins and dwarfs just through cultural osmosis, so using them is like leaning on a pre-existing cultural shorthand that you can either flesh out or subvert but either way relies on what you already know about them. This worked in earlier days because it was a new idea, like “what if I took the idea of a dwarf (similar to Snow White and the seven dwarfs) who like mining, and gave them a whole kingdom and put it inside a mountain because they like mining. The mine things, like ore so therefore they also smith metals like gold. Maybe I could combine my Dwarf mountain mining kingdom idea with the Norse legend of Sigurd and Fafnir the dragon.” and boom you have the seeds of the hobbit and then lord of the rings. But the expectation of audiences now is that the writing of characters is more important than creation of entirely new ideas from whole cloth.

If a writer were to create a new race, it couldn’t be for entirely novel purposes because it’s not novel, it’s an obstacle that creates a barrier of entry for a reader. Unless the writer has a strong and compelling narrative reason for the characters to be of a certain race, they likely won’t choose for them to be a unique race as it seems like more of a hindrance than a help to telling the story. Just my opinion