[smitty1e]

I feel as though this discussion has more to do with the control over the discussion than with the content of either phrase.

[eesmith]

Well, yes? That is, the phrase "all lives matter" seems like it's often used as a way to take control away from the discussion about largely devalued and forgotten black lives.

If you want an explanation better than "Oh well, you know", you might start with the criticism section of its Wikipedia entry, at https://en.wikipedia.org/wiki/All_Lives_Matter#Criticism .