The show was an R-rated sitcom. It was made to gear towards a late night audience. The latest "Seinfeld" aired was 9:30. This show is not for children whatsoever. The show had a very late night quality to it as well making it what it is. That's what "Seinfeld" is all about.
Seinfeld originally aired in a primetime viewing slot (between 8:00-11:00pm) on basic TV. Some could argue that now, because the show is syndicated and times vary by station. Seinfeld was never a lite night TV show. It was just your typical primetime sitcom, and that's all there is to it.