A lot of people have made the comment that most fantasy authors are white so that's what they write. But I am guessing it would be more accurate to say most published fantasy authors (in the US) are white. Like any profession in the US, whiteness has dominated for most of our history. Publishers make assumptions about what the general public is willing to read (hence buy) and exclude what doesn't fit the commercial mold. And up until the last couple of decades most books sold in the US were written in the US and the UK. There was very little commercial interest in authors from anywhere else. That is starting to change. Some of the book newsletters (buzzfeed books, goodreads, etc) are making a point of creating lists like "best books by indigenous authors" or "best books by POC". I've read some books that I probably never would have noticed because of those lists and now my algorithms are sending me more diverse recommendations. Books with POC are out there, look for them, read them, buy them, make it clear to publishers there's an appetite and you will see more of them.