Why do I only write straight white males(unless they are alien)? Quite simply, I am one and I only write characters I relate to. Can I relate to a black guy? Of course. Can I relate to a woman? Pretty sure... kinda. So why don't I write them if I can relate to them. Because I relate to my characters in a way as I want to be them. I want to see myself doing what they are doing. I see this as an escape from real life. Its easier for me to visualize a white male because I am one.
Vulps is who I am. He is the epitome of who I want to be so his physical appearance in most any form looks fairly like myself. A brown haired white male who is not beefy or fat. (I've no body weight but of course I don want my character to be a twig because... well, I want to be better and more in shape than I am.
Zarro is who i want to be in terms of business and money, but other than that, he's hopefully me when I get to be old (older than him as hes only in his 50's, I just love his old man act). his fur is grey because my hair will likely be grey(hope I still have it.) He's also an amaran, so no race there unless you want to say grey.
Darius my unused sith... may or may not be an incredibly tanned white male. In any case, he is me on my worst days, wen i feel really dark. Calculating, ruthless, and evil. I've low muse for him because I can barely relate to him.
William Seraven... He's kinda like me right now, especially around my friends. Impish, hates work, and loves to get a bit active and to escape trouble.
Why would I write a black character or a woman? I'm neither and thus that's just a bit more effort to relate to them. Can I? Yes... just too lazy and I'm used to writing only Vulps, so my characters all resemble him in most ways.
Note, my ability to relate to a black, or asian, (I'm half mexican so that one don't really count) doesnt come from my mind as I know we've the same one. Its just a visual thing. I'm not any other race so I view myself as this one, and that reflects in my characters who are extensions of myself.