Simple. Because Don King is not involved with it.
Seriously though, although the sport is suited for lower class, urban and impoverished but pugnacious people, it targets a middle and upper middle class audience that is educated, ambitious, and industrious to a point. Although a few blacks could become successful in it, it shall mostly attract a white audience and hence, white newcomers.
I know a couple blacks that watch the UFC and practice a martial art, but they are not typical and fit the UFCs target mold and only one is a good athlete.