-Varied personalities that play well off of each other
-Actually have character traits beyond "retarded father"
-Made the show unique - every fucking adult comedy cartoon has a "mentally retarded out of touch father" character; the Simpsons, Family Guy, American Dad, etc. By contrast, I can't think of any other adult comedy cartoon which focused on a group of foul-mouthed children getting into fucked up adult situations.
-Because there were four of them with different personalities, character traits, beliefs, etc. it allowed for significantly more variety in episodes.
-Actually were funny