Because it's not just a couple of throwaway lines, it's like 5 or 6 throwaway lines sprinkled in at different places in the film. It reeks of "QT thinks saying the n-word is cool."

Some of the others I can excuse as being a result of the period, or being a result of the character's villainy... yes, in RD the characters are all seedy, but they're symbols of cool, and tying use of the n-word to their coolness is dicey to me.

I never think that Jack in The Departed or Pesci in Goodfellas are "cool." Those RD guys have montages of them walking to badass music and speaking hip funny quip-filled dialogue. It's always been my biggest problem with the film.