Because if black lives really mattered, blacks would stop killing each other. Blacks would stop committing genocide against themselves by aborting more children than they allow to be born. Blacks would stop consigning generations to poverty by having the majority of their children out-of-wedlock. And blacks would stop treating education as punishment and instead treat it as an opportunity.
Until blacks start changing these pathologies, the whole “Black Lives Matter” movement, with its insistence that everyone has to change except for blacks themselves is nothing more than Progressive kabuki theater aimed at diverting attention from the fact that Democrats are facilitating self-destructive behaviors in the black community and that blacks are using the Democrat propaganda machine as an excuse to avoid the terrible (but not insurmountable) challenges that really claim black lives.
Please go read it all. Black Lives Matter is a smokescreen, nothing more.