First of all, I have never found it's use in The Wire to be excessive. I can't even remember a specific example. Incessant? Lol no. I just rewatched the show last year. They 100% don't "incessantly use the n-word" in The Wire.
Stop being obtuse. Black people in Baltimore in the 90s said the word to each other. So if you're writing a realistic, gritty show, it might include that word.
Not used by white people in throwaway jokes that are supposed to make them seem funny and relatable.
Quentin Tarantino has no reason to say it other than "it was in the script."
And who wrote that script? Hmmm.