What it comes down to is people who write the shows and sell the records know that the mass majority of the country is white, they're going to follow the viewership and money.
I love Game of Thrones myself and am fine with the way it is. If it isn't as diverse as some of us would like, then that's fine because:
All that means is you got to make something better (and I'm not talking about a Game of Thrones type of tale with colored people), and best believe you can.
Game of Thrones will not be the end all be all, just like how The Wire was undisputed for a long time.
Time goes on and the predecessor gets surpassed. The question is are you making/creating the next one? Are you doing your research to get ideas down? Been reading books, watching a lot of movies, studying your craft, getting great ideas then executing them?
The writer to Game of Thrones said himself he was inspired from previous TV shows and mentioned The Wire specifically. The directors/producers of Game of Thrones went to actors who had major parts in The Wire. Game of Thrones is a product of years of research and work by the writer/directors/producers. That's the kind of effort you gotta put in and even then it's not guaranteed, but your part is still mandatory.
Remember, people want to be apart of anything great. Look at hip-hop/R&B. Black folks keep pushing the music genre forward and people are always trying to follow that. Other forms of music have never evolved at hip-hop's pace.