Long story short, it's the evolutionary cycle of slurs. Slurs over time so they lose their offensiveness and new ones come in their place. Decades ago "idiot", "devil" or "shit" were hugely offensive now we use them as normal words. Now we are at the point when sexuality-based slurs slowly lose their power and racial slurs take their place.

My guess is that it could be partially caused by social changes - younger generations are not forbidden (and sometimes even encouraged) to openly discuss sexuality so sexual words are not taboo for them anymore so they can't offend them anymore. But racism for them is a big and very serious issue so new slurs are related to that topic. I can imagine though that in the next century when racism is not an issue anymore, people might use n-word in jokes.