Instead, I see people like you say that's "snarky" and unhelpful. How? How is it unhelpful for people to say, "We are all affected by this, so let's work together." The "black lives matter" movement drives a wedge between that chance of unity and nothing gets done because both sides are exasperated and frustrated...when they both want the same outcome.

It's quite late so I can't flesh this out as much as I'd like to, but imagine it this way: let's say there was a memorial for Jewish holocaust victims and people are talking about the terrible tragedy and how many Jewish lives were lost. Now, in the middle of this conversation, a group of people overhear it and start saying "WOAH HOLD ON! A LOT more people died in WWII than just the holocaust! Their lives are important too!"

The point of BLM is to draw attention to a specifc, and very long running instance of violence against black people. Frustration arises with all lives matter because it is perceived (rightfully or not) as downplaying those issues and trying to divert the discussion away from the violence black people face in the same way our hypothetical protestors decided that the holocaust alone was too narrow a focus and that it was inappropriate to discuss Jewish tragedies amongst others.