The same reason I don't have an Asian character in one of my own stories. "Just 'cause".
And the reason I have a Hispanic character? 'Cause I like the name Eva.
There's no real reason for the races of your characters unless your intentionally trying to be P.C., in which case it's usually completely obvious that your trying too hard. Characters should just come to you, and sometimes their Asian, sometimes their Black, sometimes their Hispanic, and sometimes their Elves, Dwarves, Gnomes, or Goblins. *shrugs*