Having watched only half a season of Madmen I don't think this will get why Madmen was popular. People liked madmen because for some it took them back to a simpler time, other liked it because of the blatent sexism (for the misogynistic to say "Hey I don't slap bitchs on the ass! I'm a good guy!" and women to be floored by how it use to be for there mothers and grandmothers.) as well as the uhh handsome actor to put it mildly. And than there was just the 50's retro look that america keeps thinking back to fondly every other decade.
70's and early 80's isn't remembered as fondly except by those that want to mock the more extreme aspects of the era. Hippies guffaw what a bunch of losers! Also cultural accepted norms of men and women had changed quiet alot, though certainly not as much as many would like. And none of those actors look particularly handsome. Though not being a Mad Men fan none of this is at all a draw back to me.
EDIT: Seriously though the 70's were really ugly aesthetically. I mean how did we go from the futuristic Falsh gordon inspired cars and cool thunderbirds and such to the massive ugly brown boxy cars on wheels. Cigarettes in tins to corded bell bottoms... gosh. The 80's were only better because it was as bad.