You know, I'm a little surprised that no one has said Scarred Lands yet. It is a great setting, one that is actually quite well suited for this path. I would imagine that Kyuss would be a great minion of Chern, or perhaps it is Chern himself manipulating events in the background.
I would guess that since Greyhawk, Forgotten Realms and Eberron are WotC properties, those are naturally the most common settings used. Avandu, Scarred Lands and other third party settings have less exposure and may not be suited for the adventure path series. Or, perhaps, not even for Dungeon adventures.
I find my homebrew more interesting than published campaigns, mustly from the history that it has for me. I'm sure that for Joe Gamer GH or Eberron would have more detail, but I've been running this campaign off and on since 1983, about 10-12 years of total time at a game per week. Eberron really interested me, but my own creation is the closest to my heart.