>>211730290Cause it was novel back then to do so. Cartoons were pigeonholed as children's entertainment for the most part, that was at least certainly the case for TV animation. The Simpsons really opened up TV animation to new possibilities, albeit they did this with a decades-old TV format: the comedic family sitcom. But it was the irreverent and sarcastic tone of the Simpsons which got everybody's attention. It was, at the time, considered "subversive" in its apparent criticism of family life and for its sardonic social commentary.
So going into the 1990s you have this explosion of animation, a lot of it airing on MTV or other cable TV networks with laxer standards for content, that tries to be different from sterile children's TV shows. Sometimes you got Ren & Stimpy or South park with their gross out style and humor, sometimes you got shows like King of the Hill or Daria which reveled in the mundane, and showing the absurdity of normal everyday life.