But how are those murders treated?
It’s exactly the same reason we Catholics protest abortion (to the point that some would consider us overly obsessed with it) but don’t usually feel the need for organized protests about murder in general.
The murders that the wider society seems not to care about or even to praise (abortion in our case, police murder of unarmed black people in BLM’s) are the ones that need special attention called to them. The murders that are appropriately punished are of lesser priority, even if more numerous. Yes, it would be good to reduce those numbers in general, but they do not pose the same issue as the murders many people don’t even count as murders.
There are, I’m quite certain, people working within black communities to reduce “black on black” crime. But they don’t need to draw everyone else’s attention to that in the same way that they do murder under the color of law.
In the same way, I’m sure there are Catholic organizations that work to reduce violence and poverty and other sources of unjust and unnecessary death. But we have organizations and marches dedicated to one particular cause of death (abortion) and the unusual group it targets because it needs special attention in the present world. Same with BLM.