My post on this very topic is called Representations of Class on USan TV. Some of the points made in that post and subsequent discussion that have not already been covered:
1. USan TV is extremely visually-driven, very obviously as compared to Brit TV. Hollywood directors, set designers, and show-runners are really invested in making their shows *look* interesting, pretty, or visually striking -- and this is in fact a considerable part of the international appeal of USan TV & movies. Pictures need less translation: for a while, "Baywatch" was the most popular TV show in the world, because it required no translation (or detectable scripts) at all.
2. The visual, superficial, money-is-no-object approach is, as slashpine notes, a very good match to the ads, and selling ads is what USan TV is *for*.
3. Reality TV is filling the gap. American Idol in particular is structured and edited to be a set of rags-to-riches stories, or more precisely working-class-to-fame stories. The American working class is, generally speaking, more interested in following such stories than in watching realistic observation of their own lives -- and these stories are also a better match to the ads than realistic ones would be.