It's not just history, it's still in the present
Also, there hasn't been that much for blacks to move on to. I mean honestly, the blacks still face difficulties in America today even if it's subtle. A lot of the white social construction from the times of slavery and of the Nadir of the early 1900's where lynchings were rampant still emanates into today's society. So it's not just that blacks are giving the word meaning because they react to it, it's because whites still give it that meaning by how some act today. Honestly, America is not equal, so racial slurs still hold a bearing!
Also to put it simply, it's easy to say not to react to say to not react to a word or to just brush it off when your race hasn't been subject to the same trauma so recently. Whites really don't understand how easy they have it in America. They've always had others just like them in most schools, jobs, and social settings. Whites rarely are the 'odd' person out in America. White perspective is lacking on these matters, and trust me I know because I'm mostly white and am part Native American but still, mostly white so I've felt that lack of perspective the more I have learned about how much slavery and post-emancipation has impacted America's present.