I'd guess that they can reach a lot of people through recordings that wouldn't go to their concerts, presumably because of how white people are scared of black males. Probably not as many people are scared of Billy Joel fans.
Maybe it's an issue of fear, but I doubt it; probably a combination of cultural stigma (thinking it is "weird" for a white person to go to a rap concert) and relative wealth of the target markets (concerts are much more expensive than albums, especially when you include travel costs).