A pretty great statement really up until the slogan "All Lives Matter," which is as polarizing to some as "Black Lives Matter" is to others. Trying not to get too political in this stream, but this opens up a big can of worms.
In reality, both statements are true. Indeed, one cannot exist without the other. That's looking at both statements in a kind of philosophical vacuum. But there's also a social context behind both statements that is unique to our society and our moment.
A bit of history: "Black Lives Matter" came first, founded in 2013 in response to the acquittal of George Zimmerman, police brutality, and other issues. https://en.wikipedia.org/wiki/Black_Lives_Matter
Someone coined the term "All Lives Matter" shortly thereafter as a kind of backlash, and it caught on. However, it's worth noting "All Lives Matter" isn't a real organization and it doesn't have any discernible agenda behind it for the advancement of anyone's rights. The spirit behind it isn't objectionable. But in practical terms it's, at best, just a slogan people say to make them feel better about not doing much of anything, or at worst, to actively oppose BLM's goals. https://en.wikipedia.org/wiki/All_Lives_Matter
The Black Lives Matter movement never meant to imply that other lives don't matter, and it still doesn't. But when some folks started shouting "All Lives Matter" in response to every invocation of "Black Lives Matter," they sought to imply nefarious goals to BLM that never existed.
But don't take it from me, take it from Obama:
--"President Barack Obama spoke to the debate between Black Lives Matter and All Lives Matter. Obama said, 'I think that the reason that the organizers used the phrase Black Lives Matter was not because they were suggesting that no one else's lives matter... rather what they were suggesting was there is a specific problem that is happening in the African-American community that's not happening in other communities.' He also said 'that is a legitimate issue that we've got to address.'"
tl;dr: saying "All Lives Matter" at a moment in time when black people are fighting for their rights and grieving over unjustified police murders and similar injustices can come across as tone-deaf.