Why do we find offense at people using the “N word” but not the “R word”?

From 1999 through 2013, the Native American population of the United States experienced nearly the same rate of deaths at the hands of law enforcement as did the African American population, the difference being there are approximately 5.1 million Native Americans in the United States while there are around 45 million African Americans. If you do the math, their percentage of deaths based on their own population is higher than that of African Americans. Yet when someone tries to say #NativeLivesMatter they are ignored, mocked, or yelled at by the BLM movement (black lives matter) saying their lives matter, but they aren’t important enough to be addressed right now.