No one said it did.
However in many parts of the world, especially in America, it seems as if black lives are the only lives that doesn't seem to matter. It is actually a shame that we have to remind people that black lives matters as much as everyone else's lives, because it still shows that racism is still a big problem in the world.
Yes of course, ALL lives do matter, but they do not have to be constantly reminded this. Why? because black people are the ones that is treated as an outcast in society. Have you not read about the case of Sandra Bland? Things like that are the reasons why we are told that "Black lives matter"... Because they do... as much as every one else's.