Because I colored their skin with the same color? Well... I sort of imagine Twilight could very well be asian, but race wasn't really something I put much thought into. I just didn't really imagine any of them as black or hispanic.
There's nothing wrong with them being white either. Plus, there are no dominant browns in any of their original pony color pallets and a lighter skin complection throws more attention on the colors of their hair, eyes, and clothing, which correspond to the original colors and I wanted those to stand out more since they're the originals. I would have made their skin the color of their pelts, but then Rainbow would have had blue skin and Pinkie Pie pink skin and so on and that would have looked silly. White worked better with the color pallet, so I went with that.