The real issue is that, while strictly true, "All Lives Matter" is a tone-deaf slogan that distracts from the real problems black people in America face.
The best explanation we've seen so far comes from Reddit, of all places. Last year, in an "Explain Like I'm 5" thread, user GeekAesthete explained, clearly and succinctly, why changing #BlackLivesMatter to #AllLivesMatter is an act of erasure that makes lots of people cringe.