Because the black homicide rate is really high. It can be a bit easy to forget this, caught in the abstract. The movement isn't called "the police shouldn't kill people" - it's called "black lives matter". I'd expect a movement named "white lived matter" to focus on all-up issues for white people.
> black people are twice as likely to be killed than white people by our murderous police.
Among young people (18-45), black people are ten times as likely to die by homocide (27.6% of deaths vs 2.8%). From the [CDC](https://www.cdc.gov/healthequity/lcod/men/2017/nonhispanic-w...).
Giving yourself a broad focus "black lives" and then ignoring the number one category of death among young males (homicide) - it's a bit odd.
Imagine if you ran across the "Cancer Matters" institute and it turned out they don't study lung, breast, or prostate cancers (the three most common and deadly). Instead they study just one specific subtype. What would you think?
Edit: just a small comment, but I'm more trying to "explain the argument" than make it myself. Asking if organizations are "pure enough" to their purposes is a common tactic on all sides, and it's clear that _some_ who ask this don't do so in good faith, trying to set the bar impossibly high. People have wildly different perceptions of BLM, in part because BLM is different organizations in different cities. I don't mean to endorse those impossibly high bars, only to _explain_ that the criticism is that BLM is not meeting the bar to some folks in terms of actions matching the mission/label. I'm trying to answer in good faith - assuming that the question was asked in that same good faith - as a legitimate request for explanation.