Because no mainstream white would dare do so under the present stifling PC atmosphere. Imagine if the races had been reversed in this case, with an attractive black couple as victims and a gang of white hoodlums as the perps. It would have become not only a national news story, but a subject of political debate as well. Obama & Hillary would have been all over it. Bush & McCain would have both denounced the heinous crime. Every black preacher and civic leader within 100 miles of Knoxville would have led a march through the city in protest. Tennessee's governor would have attended. The mayor would have apologized for not doing more to prevent this from happening ahead of time. And so on.
But there is no legitimate representation permitted for whites as a group. If a white city councilman or minister had tried to make an issue of this, he'd have immediately been attacked by every media outlet for inflaming racial intolerance. His fellow pastors or politicians would have ostracized him. So everyone just keeps quiet, because speaking up can result in a loss of respectability. The only ones who show up to protest are white supremacist types who, for good reason, have no respectability to worry about.
Ping!