Fat girls can be mean to skinny girls, but skinny firls cannot do the same to fat girls.

Racial issues nowadays are always looked at in the light of previous experiences and cultural norms.
Historically there has been a lot of violence against blacks. We're supposed to be past those transgressions of the past, but every incident brings back memories of the bad old days and triggers a strong reaction. Everyone is aware that blacks were treated very badly in the US up until not too long ago. And white people were in general not the victim of racial violence.

So every time something like that happens, the incident with a black victim gets massive feedback due to the past when that violence was institutional and socially acceptable. Racial violence against white people was not, so every incident there is lacking that strong social feedback.