I’m a bit confused by your question, perhaps due to the lack of a description or tags.
If you mean “why do people say the phrase six feet under?” Then my response is because the typical depth of a grave is 6 feet from the surface.
If your question is more along the lines of “why choose to be buried (six feet under), versus other means cremation or perhaps above-ground burial (as in a mausoleum)?” then my response to that would be people choose to honor traditions of their ancestors and local custom, while others have religious reasons for choosing burial over other means.
Finally, one of my all-time favorite dramatic television series was Six Feet Under, so if that was the question, my response would be that Alan Ball, the series’ writer and creator, also famous for the Academy Award-winning screenplay and film American Beauty, crafted a fascinating, fresh storyline that was centered around the lives of family members running a family-owned mortuary business. It was an awesome show.
Now having said all of this, which one was it?