With Eminem, I think Dr. Dre got wind of the fact that about 80 percent of rap's audience is white teenagers (when it comes to the mass-marketed stuff) and decided to tap into it. All he had to do was find someone with more street credibility than Vanilla Ice who happend to be a really good MC, and then promote him like crazy.
Although virtually all the significant advances in popular music come from black people (rock, blues, jazz, reggae, soul, funk, metal [attributed to Hendrix], hip hop, reggae, ska, R&B, etc.), the record companies seek to market whatever comes along to the white majority. That is because the biggest portion of purchasing power lies within the majority's hands.