Why? Obviously because the light-skinned girl has all of the right qualities for the black male (the wavy hair, the light eyes, etc.). She is close enough to white without being controversial. The light-skinned woman has just enough melanin to have a bit of kink in her hair and sway in her step. This keeps anyone from starting the interracial couple conversation.
Members of the African-American community, and only members of the African-American community, praise light-skinned women. And it's not like we're just noting their beauty. Because that'd be okay. But we are also shaming women with darker skin. Their skin is "too" dark; their curls "too" tight. We are treating them as if they are less than. Young black men will discriminate against women that have the exact same skin tone as their mothers and sisters. And it's sad.
Being an African American woman, who is not light-skinned, I can honestly say that my brown skin is a burden. I am never beautiful; just "cute for dark-skinned girl". My hair is never "good" hair; just "pretty good for a dark-skinned girl's". My best will only ever be mediocre in the eyes of other black people, simply because of my skin tone.
I don't know when or why dark skin became something to hate, or to be ashamed of. (Especially when white people tan, but we won't get into that.) And I definitely don't know when or why it was okay to divide ourselves based on skin tone. If I see another "#TeamLightskin/#TeamDarkskin" or "brown skin girls are winning" tweet, I may lose my mind. No other race in the world has created distinction within itself based on skin tone. It's pointless. Especially when we're all the same to everyone else in the world. Other races don't really see a specific tone, all black is the same to them. So why isn't it to us?
Hopefully soon we'll realize that we're all beautiful, light-skinned and dark-skinned. Beauty comes in all shapes, sizes, and colors. It's based on so much more than the pigmentation of your skin. Your skin tone will only ever determine your beauty or worth if you let it.