My bet on why PACG blew up is because it filled an otherwise empty niche. It's this niche that I, and my group of friends, found this game to fill. That niche is a middle-ground between board (or card) game and full-blown pen & paper RPG. As experienced gamers (of all types) we've played Dominion, Catan, etc, and wanted something a little more complex, but no one wanted to invest (time or money) into a D&D or Paranoia that required serious dedication as well as a DM/GM.
Although there are other attempts at making "shorter, card-based D&D" with games like Thunderstone, they all lacked the impetus of serialization (i.e. continuous stories / campaigns) that kept peoples interest.
Considering how long it took us to finish the Runelords campaign (note: very long), this level of required dedication is perfect for us while maintaining our interest because of developing individual characters.