Compare the size of the ships John. Not sure with Princess ship you are discussing here, but they used to sail the smaller (and older) ships up in Alaska. RCI has a large ship with more and newer amenities, which they tend to price higher. Its the same elsewhere too. The larger/newer ships are more expensive to cruise on because people are willing to pay more for them.
Now, I could be all wet here. If you are not talking about the smaller Princess ships, then I can't explain it. (Other than the statement made by the CEO of RCI in that he "won't discount cruises to fill ships")
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007