That’s because the ethnicity of a criminal is usually not a relevant detail. It is not similar to the Japanese media hiding the motive of the assassination.
That’s a big assumption rooted in a moral framework rather than in objective science.
Or for this case, lets say the perpetrator was of Chinese origin instead of Japanese. Do you think that wouldn't be relevant at all?
Come on, don't be coy. There are many things which aren't relevant like where the criminal is from or how old he is.
The reason they hide the ethnicity is because it helps racists to profile.
The problem is that they go too far the other way, like labeling Kyle Rittenhouse, not just white, but a white supremacist without any evidence.