Because that catchphrase started in reaction to Black Lives Matter, and is used largely to signal opposition to the goal of constraining police actions. If people were holding All Lives Matter protests in opposition to police violence of all kinds, I doubt they'd get much flak; instead, they're protesting the idea that black lives matter.

Just because you say it means something to those people, doesn’t mean it means that to them. You don’t get to choose the hidden meaning behind other peoples words, and you’re clearly giving them the least charitable interpretation