Just about all races (when they date interracially) tend to desire white partners. Biracial men can probably date a white partner easier than black men, so tend to do it more often. That's my guess. You could consider my multiracial, being of Black, German, Italian, and Cherokee (though I Identify myself as Black) and I've dated Black girls, and would do it again. I don't really care much about race.
Some younger bi-racial males tend not to date black females because black males hate on them more if they do. In other cases if the mother is white the reason could be Freudian. As a bi-racial male myself I'd date anyone attractive enough, but sadly enough I was too broke to date the better looking women of either race black or white & found myself with Latina women. I still think most women care more about money than race.
I'm a mixed black/white male, and my reasoning is psychological. Black girls used to hit on me all the time in school, like telling me I'm so good at basketball when I've hadn't made a single point. I could empathize with girls who get hit on nonstop by rude guys, and my friends (of all races) would make fun of me because black girls would practically stalk me. That may have inbedded in me a disposition towards not being attracted to those types of women. To this day black women are always trying to talk to me, but never any other race. Maybe I go for non-black because it's more of a challenge. In terms of school, black women for me are like a blow off class where I know I can ace the class without effort, but I'd rather take the Honors courses.
I understand your concern but from a biracial guy's perspective its all about physical attractiveness and black women are not very strong in that area. Its quite simple really. He chooses white or biracial because they fulfill his attractiveness standards that's all
My theory is if that they were raised around predominately white people, they take on the white culture and want to assimilate and think white girls are better. but the ones I know that were raised around the black culture prefer black girls. just so happens that most biracials are products of white moms and black dads so the mom and her family will probably have the most influence