Kids in my generation were heavily into rap and r&b, so I don't think it's a modern phenomena. Even in the early 90s, the overwhelming majority of hip-hop consumption were from white suburban males. It goes back a while, and you might even look at the popularity of jazz and disco music as the predecessor to all that.
But as to a reason why, I don't know, but I have a theory. Just like science fiction and fantasy and comic-book stories appeal to younger audiences, I think black music can play a similar role where it captures foreign environments, cultures, and norms and makes it accessible through story-telling. It's the appeal of the distinct and different that fascinates them.
There's also subversive elements in rap music that appeal to teenage rebellion much in the way that punk and rock music used to back in the day, but rap has grown far more mainstream today than in years past and is the genre du jour for this era.
