most the time is because some artists have a hard-time getting the skin tone right. I tend to try and draw my characters with medium skin tone, but drawing someone with an ethnic background is quite difficult to get right. You have to study faces of other backgrounds to understand their facial features and what not, its not just about their skin tones, as other races have different eyes, jawbones, cheeks, lips, noses etc.
drawing a person from a white background is much easier than someone of an ethnic background.