The official Black Lives Matter statement:
#BlackLivesMatter was founded in 2013 in response to the acquittal of Trayvon Martin’s murderer. Black Lives Matter Foundation, Inc is a global organization in the US, UK, and Canada, whose mission is to eradicate white supremacy and build local power to intervene in violence inflicted on Black communities by the state and vigilantes. By combating and countering acts of violence, creating space for Black imagination and innovation, and centering Black joy, we are winning immediate improvements in our lives (Black Lives Matter, n.d.).
There are analogies you can use to explain why "All Lives Matter" misses the big picture surrounding the BLM movement. The "All Lives Matter" argument isn't inclusive or helpful:
From the BLM website:
We affirm the lives of Black queer and trans folks, disabled folks, undocumented folks, folks with records, women, and all Black lives along the gender spectrum. Our network centers those who have been marginalized within Black liberation movements.
We are working for a world where Black lives are no longer systematically targeted for demise (Black Lives Matter, n.d.)