There are several music genres in the US which find its roots in the African-American community such as Blues, Jazz, Rock and Roll, Rap and Hip-hop. Why is that?
This is such a huge question that it'd take books and books to answer it, but I've sort of summarised the different sides of it here. There's also a bunch of other answers on my flaired user profile related to this topic which you might find interesting. Hope that helps!