It's critical nostalgia, at takes shots at the idealized typical American values family stuff, which just about makes the critics swoon. They still like to believe that attacking traditional ideals or values is something unique or interesting or courageous, when in fact, it is practically a cliche in Hollywood by this point. I mean, the why of that mentality is kind of obvious, since normal people who fit in and get along are seldom inspired to strike out for Hollywood to try to re-write the world as they wish it to be, but you'd think they'd have recognized just how trite this sort of thing has become. And since it's set in the 60s, they can fight the favorite battles of liberalism all over again, or else plausibly portray the standard liberal icons as underdogs and martyrs.

All this stuff, plus the acting talent, depth of writing and production values are a lot more interesting to critics than a story anyone can understand. If anyone can understand the story, after all, why would we need critics. Mad Men is like their own private party, where they can congratulate themselves on being among the elite who "get it". In fact, a lot of people get it, we just don't see the need for it.