"All Lives Matter" redirects attention away from the specific thing "Black Lives Matter" was meant to draw attention to, which is that, if you are black specifically your chance of getting killed in an interaction with police is higher. There are other racial issues surfaced by black lives matter, as a slogan, eg, having to do with health care, real estate, etc.

No one things that all lives don't matter, but the statement "black lives matter" is used to highlight specific social problems in the US. It would be like if you cut your finger and asked for a bandaid for your finger, and someone kept saying, "But ALL your fingers are important."

It was a bad faith argument to shut down discourse about summary executions of black men in the streets of the USA. It's been about 10 days since they executed a white soccer mom who had not committed any crime, the people who were harping on about 'All lives matter' are still no where to be found. The whole time it was empty rhetoric, people who care about human life understand that sometimes there's a group in particular being oppressed.

Because it was created as a reaction to the Black Lives Matter movement and was designed to imply that the existing status quo, where the USA acts like black lives don’t matter, isn’t a real problem.

Only legitimate crazy people here would have a problem with the sentence “All lives matter.” People have a problem with the slogan “All Lives Matter”, which was made in reaction to “Black Lives Matter”.

I don’t know if you are ESL if you aren’t American but the capitalization I used matters and it’s difficult to explain the context of hearing a sentence vs a slogan. That’s also before you get into people specifically trying to conflate them so they can do a sort of motte and bailey type argument by claiming they weren’t saying “All Lives Matter” just that “All lives matter, so they can uno reverse you as the bigot