I really don't understand why "Settlers of Catan" hasn't caught on more in the U.S.
What does "caught on" look like? Every tabletop gamer I know has a copy. Settlers of Catan is as hot a game as you're going to find in the U.S. The ceiling on Settlers of Catan is that most people don't play board games.
There are a lot of non-gamers who own a Monopoly, but they don't play it much. With the right marketing push, you might be able to get some of those people to buy a Settlers and stick it in the closet unused with their similarly-unused Monopoly board, but for the fact that it costs almost $40. Milton Bradley/Parker Bros. games can be had for under $20.
This really is the same issue with $1 phone games vs. $50 DS and PSP games. Lots of casuals who have low demands, but high numbers vs. the few connoisseurs who will pay for something they actually use and enjoy with more intensity.