Black Lives Matter is a response to how black people in the United States seem to unfairly be the targets of violence, and grew into a big international movement. They have succeeded in some goals, and their ideas aren’t bad, it’s just that they could be much more effective.
The problem is a question of semantics. Allow me to make a few comparisons to illustrate my point. If you say that you like the Green Bay Packers, it is implied then that you don’t like the other teams, or at the very least that you like Green Bay more than other teams. If you say that you like the color red, it is implied that you like it more than other colors. In both cases you aren’t explicitly stating that you dislike other colors, but the implication and understanding is very clear. Why else would you say that you like red, if you aren’t stating that you like it more.
So back to this movement. When you say “black lives matter,” intellectually we know what you mean, but the immediate instinctual understanding is that you’re saying “black lives matter more.”
Now, social media campaigns and hashtags don’t really do anything for anyone. The best you can hope for is to get the attention of someone who is actually going to do something. But, when your slogan, the first (and usually only) thing that people hear, is excluding literally everyone else, you aren’t likely to see much help. And that’s why you hear a lot of people responding to this with “well, no, all lives matter.” And then the whole idea is dismissed, and the campaign does nothing.
What might have worked better is a small, simple change. Something like, “Black lives matter too.” The addition of that one word completely changes the implication. Now, it means “Other lives have always mattered, but black lives have been getting the short end of the stick, and we want to do something to help.” This is a much more inclusive slogan than “other lives don’t matter, only black ones do,” and is much more likely to garner sympathy.
Nobody likes injustice and racism. People want to help, and fix problems, but when you make them feel like you don’t want their help, they aren’t likely to do anything for you anymore. I’m going to paraphrase an old episode of Law and Order, following a high profile case. Legal equality has been achieved. Under the law, genders and races are all equal, and there are no laws that disadvantage certain groups (note: this is actually not entirely true, as affirmative action is a terrible idea, but that’s another topic entirely). But now we’re in the hearts and minds stage. We’ve changed the laws, but changing the hearts and minds of everyone is harder, and takes time.
That’s where we are right now. You need to change people’s minds (or wait a few generations until the racist people all die out, but in that case you still need to change the minds of their children). But you’re not going to get there by excluding the people whose minds you need to change.