My reason is simple: every major character I've created so far, with one possible exception, is white, and I don't want people getting the wrong idea about me.
(For those wondering about the possible exception, it takes a little explanation. The voice of Frylock from Aqua Teen Hunger Force? That's what I picture Raff from Stress Puppy sounding like. The voice may be a black man's, but Raff is a golden retriever, not a human, so I'm not sure if he would qualify as an actual black character given the universe he inhabits. Besides, comic strips don't have "voices".)
Each of my black friends that I mention this to looks at me in surprise. The reaction is usually along the lines of, "Wrong idea? You're just creating what you're familiar with. You're not racist. Anyone who talks to you for more than ten seconds will realize that. Why would you think anyone would get the wrong idea?"
The reason is this: there is a web site claiming that My Little Pony -- Friendship Is Magic is actually a metaphor for white pride.
No. I'm not kidding.
The site points out that Ponyville is modeled on a Western European village, like in, say, Germany, with environments, structures, layouts, and shops reflecting that. "It teaches that there is nothing wrong with being white! It's just too bad this important lesson has to be taught subversively in a kids' cartoon instead of out in the open!" And keep in mind that, whenever someone draws a human version of Twilight Sparkle but depicts her as black or Asian, bronies start raging that people are misrepresenting the character.
If people can misinterpret a cartoon about candy-colored ponies THAT fucking bad, it scares me how they'll misinterpret my stuff. I'm trying to nip this in the bud.