The same reason why I have any other preference: because I just do. Light haired women have just never done if for me. I've never understood the obsession with blondes. Dark hair just looks healthier and more sophisticated.
Hm... I actually have no idea. LOL. Tall, dark, and handsome maybe? xD Well... in general, I love, love, love Asian guys, so maybe that's why... Asians almost always have naturally dark hair. And I like asians. Therefore.. I like dark hair? That's the best explanation I can give... I apologize. :P