Here's your explanation:
Mad Men's success is all about timing. It premiered just as The Sopranos and The Wire finished up, and Lost started sucking. Modern HBO/Showtime juggernauts had yet to begin, and this was the first of the AMC originals. There was really no 'great' tv to pick up the slack, so 'very good' had to do. There was about a 2 year period where Mad Men and Dexter were best things on TV. The fact that it aired on basic cable instead of HBO had to help.
About the show itself: There are 2 situations where I explain something I enjoy, and have to start with the sentence, "you're going to think this sounds really boring," those two things are the Harvest Moon series of video games (farming sim) and watching Mad Men. It's pretty much the opposite of The Walking Dead, where average-at-best actors and bad writing are coupled with incredible source material, great storyline progression, and shocking visuals. Mad Men is all about character progression. You could move the setting from Ad Agency to Law Office or Senate Floor, and it wouldn't change much. Also, like most modern shows that last 8 seasons: A) You have to start from the beginning B) The last few seasons haven't been great, but I must watch till the bitter end.
I leave you with Pete Campbell falling down stairs: