Let us not forget that TV shows have to satisfy an audience as well as fit in with the situations in the show. A lot of viewers (myself included - hey, I was in 6th or 7th grade when the show began!) watched due to the sexiness of the two stars.
Clark got to wear the Superman suit regularly, which was revealing in its own right. With a few exceptions in which she wears gowns or disguises, Lois is usually seen in her "work" apparel. Yes, shorter skirts may have been "not work appropriate," but I think the show's wardrobe department was likely trying to satisfy us at home. :p
