Most white girls have thin lips. Less lips, more mouth.
If you want to get technical, it's probably do to the fact that little black girls were told from an early age that "sucking dick is bad.. only trashy white hoes do it". So we think of it as "Ew, I'm not going to put his penis in my mouth! Gross" and if we DO decide to do it, sometimes we may not do it as well just to point out that we're 'good girls' and its only our first time doing it. I mean, that's how I grew up.
Any more questions you have? Ask Lindsay Knowhan. She Knows it all.