It's not popular because people haven't given it a chance.

Part of the problem might be that the type of person the show would appeal to, the "Good TV" demographic, is the exact opposite type from those depicted on the show.

Mad Men seeks sophisticated viewers while portraying sophisticated characters. The Wire and The Sopranos were about outlaws, and theres something very anti-establishment about being a Good TV person. The "Good TV" demographic also views itself as superior to the Bush-Voting morons who watch the crap on the networks, so having a show be on a premium channel appeals to these snobs.

Peter Berg has also said that alot of FNL fans are young dudes who probably just download it. Thats what i do.