I don't know, but I suspect it is because they tend to be longer, so the per diem rates are lower, but the overall cost of the cruise is about the same. They also have a lot of days at sea and fewer port calls, and those port calls aren't always at "standard" ports. In the end, I guess the passengers are doing the line a favor by being on board and spending money when the ship could be sailing empty. I would think that they would sell better (they're a favorite of mine), but if they did, I'm sure the prices would be adjusted accordingly.