Class issues were brought up in All in the Family and even Good Times , but these were subplots, rest areas to attract new demographics to a network freeway of billboards and convenience stores of capitalist life. Other than this rare bit of social commentary, classic sitcom gags were primarily based upon physical humor, The Honeymooners for example. The only physical humor today, however, is of bodily functions, what things come out and especially what goes in. Viewing the totality of a sitcom's lifespan, not much goes in or out. Characters always operate on the same level, a couple in love remains so, with a few detours thrown in just to make sure the audience is watching.
Characters are figureheads, ideologues who never change in any fundamental way.
One week's crisis is forgotten by the next. Any hope for evolution from the cultural climate in which it originated to a more current one is only addressed in the wardrobe department. On some levels The Simpsons used to achieve this, but a cartoon has an entirely different set of rules. Destroying traditional functions of television is a much easier conquest when the characters are animated. The one notable example from the last decade that subverted television's rules while maintaining a fanbase is Murphy Brown .