Because that's what you're gonna get if you keep letting police officers and vigilantes gun down black men, children, and the mentally ill. To borrow a saying: fix it now.
Obviously the lives of some people are worth less, a lot less, than others, and it's not surprising who ends up being the victims.
I'd love to blame this on conservatives, Republicans, etc. but this is a bipartisan problem that's built up over many, many years. Something radical is going to happen, and if it isn't radical change in policing methods, and change in the methods of policing the police, then it will come from the people, and it may not be controllable.
Let me make some modest proposals:
1) Decriminalize all the bullshit, such as the petty offenses that Ferguson turned into a money factory, and occasioned the killing of Eric Garner (sale of untaxed single cigarettes -- the horror) and too often serve as the excuse for oppression;
2) Demilitarize the police, which harms the relationship between the police and the citizens, even if not involved in any particular case;
3) Scrutinize the police, starting with mandatory dash and body cams.
4) Reallocate resources (hey, jail time ain't solving the problem) into community courts and similar endeavors.
5) Construct an adequate mental health care system.