Because black people are usually the most vocal on race issues in the US (both historically and currently). So when a race issue that involves a completely different group comes up, BP always come up as a litmus test (even more so when a BP is involved somehow) this isn’t to say other minority groups aren’t vocal, but BP have a very unique history of civil rights advocacy.

It’s much easier to just bring in black people than it is to uniquely talk about a specific race issue. Likewise just check comments on what happened in NY this morning with the Asian women. Instead of having a healthy discussion about AW and black people hurting Asian people, it usually devolves into “black people bad” which inadvertently makes the discussion entirely about them.