Because the majority of gamers are white, especially American, gamers. Not to mention the majority of devs are white as well.
However this isn't to say that other audiences aren't supported. I mean Japanese gaming audience is widely supported as well. But it's the next most abundant race that plays games. It's really just about where the developers are located as it's a culture. White American developers are going to make games catered to themselves haha. Japanese developers are going to make games that are their culture. Just need more devs of their own race to create games more catered to their culture I suppose.
However I don't see how you can cater to a certain race considering we are all human beings that share the same needs anyways. Differences in cultures due to race are so tiny that saying a black person can't play a game with a white character is silly. Normally most of your cultural differences will be based on nationality. Which is why Americans tend to like shooters, and Japanese are more into RPG's and fighters.
