Because BLM is about the low tariff US society at large places on black lives, illustrated on a weekly basis by state (police) killings which simply shouldn't happen.
Black on Black killings are not directly symptomatic of a societal bias against people of colour. Whilst being black might mean they are statistically more likely to be caught up in gang killings, those killings don't happen just because they are black. Unlike the extra judicial killings at the hands of The Law, which are often rooted in the institutional mistrust of the black man.
Black lives matter isn't so called because they seek to save all black lives. It's mo-op is to level the playing field where white faces otherwise get better treatment (like being given a similar benefit of the doubt before being 'suicided by cop')
It's not often I use this phrase in GD, but that is a great explanation with good reasoning. Good stuff cheeseface
