Most of the Caribbean sailings are also part of larger, B2B or Collector's Cruises. When the inventory falls out of balance between the various legs of the cruise, you occasionally see entire segments become closed out even though there are rooms left. It could be that someone was working on the sailing but still has to finish up. If that's the case, you'll see rooms again Monday or Tuesday.
On the other hand, it could be sold out. We are on the Nieuw Statendam on Dec. 10 - 17 and that's usually a soft week. It's completely sold out though.