It's pretty simple. Most authors and sci-fi and fantasy for the past 70 years have been white and default white characters. However, you can go looking and find plenty of authors who write wonderful stories with characters of all races. Octavia Butler has wonderful stories featuring majority black characters, some sci-fi, some post-apocalyptic America, not sure about fantasy for her.

It is good we are seeing some more movement in that area in more modern fantasy and sci-fi. And it's one of the reasons I think it's great to see fantasy works that are brought to television using more multi-racial casts to make the world more accessible for more people.

It's the same reason a lot of books have mostly male characters. If an author isn't being thoughtful about character creation they just go to their default and white men have dominated the industry for 70 years and their default has been white men. I don't like calling it racist as I don't think it's intentional I think it's just thoughtless