Okay well I'm not a guy so can't give you that perspective, I'm white and female, but for what it's worth this is my take/experience:

Some people care more about outer appearances than others, they have a 'type' and do stick to it. I was once told (when young) by a young man that he thought I was nice but he preferred blondes (I'm dark), fair enough, saved us both time. I don't find people who are so rigid and superficial very attractive either!

So maybe some guys only date white girls, that sounds quite rigid and superficial to me, a bit boring perhaps?