Because it doesn't tap into the same markets, nor does it have the penetration. Look at it this way, black on black violence doesn't excite or agitate the white population the same way that interracial violence does. Black on white crime doesn't interest the black population at all and it doesn't tap into hot button topics like white privilege or white guilt. But white on black crime? It incites riots in urban black populations and brings out the mouthpieces to vacuum up the cash. It taps into the aforementioned hot button issues and creates plenty of sub-plots for pop culture icons and pop psychologists to exploit. As an added benefit, it enrages right leaning people that black on white crime doesn't get the same media attention, which adds even more fuel to the fire.
In other words, it literally hits every target in a single story. That's very cunning market exploitation, which is exactly what the media does best.
