Because all hell would break loose if the general American public understood the magnitude of the problem of black violence and crime. It doesn't align with the interests of political or media elites to demonise blacks, while they rub their hands gleefully over the prospect of an evil White shooter.
According to Wikipedia, 4 of the 6 sources for definitions of mass shootings in America specifically exclude "gang" related shootings. https://en.wikipedia.org/wiki/List_of_mass_shootings_in_the_...
How convenient.