To understand this phenomenon you have to first understand how black on black crime becomes a part of the discussion. Let's run through the prototypical situation. A black person is killed/harassed by a cop. Black people get angry and demand that the police treat them with respect. Furthermore, Black people extrapolate that situation and say that the U.S. is still racist (which it is). The accusation that America is still racist makes White people defensive. In an attempt to win the argument, White people then decide to attack the argument that Black people are making, not head on by addressing their claims of police brutality but rather from the side by saying, "If you really cared about Black lives, then you would be more concerned about Black on Black crime rather than Police brutality because Black on Black crime affects more Black people than Police brutality."
The people who use the "Black on Black crime" argument will never say that police brutality does not exist because the proof is in the pudding. What they will do though, is claim that Black people are more focused on the last vestiges of racism rather than truly bettering their community. You see, at the root of the "Black on Black crime" argument is the fundamental belief that the cause for Black people's troubles is themselves rather than America as a whole. Now that you understand why some White Americans raise the Black on Black crime argument, hopefully you can understand why the phrase "White on White crime" will never become a socially acceptable phrase. In America, White America = America. If there were high rates of crime among White people, the media would just say that the entire country has a crime problem.
While people who bring up black on black crime say that their doing it out of the compassion they feel for African-Americans one only has to look at the context of when they bring it up to understand their true motives. What I mean by that is if a White person bought up Black on Black crime randomly and said something along the lines of "The crime rates in Black America are sky high. As a society and a Country we need to do a better job of providing them economic opportunity because crime will spring up in areas where their is no hope." Instead, though, most people who bring up Black on Black crime only bring it up to turn the discussion away from Police Brutality. A healthy functioning country that has moved beyond the great stumbling block of race would have many people who were doing the former and few people doing the latter.