You might wonder why many cruises to or around Hawaii involve a 40 hour each way diversion to Fanning Island. The answer lies in the legal regulations involving cruise ships, the United States, and our minimum-wage laws.
There are only three cruise ships flying an American flag. These are Norwegian Cruise Line's Pride of America, Pride of Aloha and Pride of Hawaii. Even these three ships are very new creations. Before they came into being, no cruise ship would fly an American flag. To do so would mean they were subject to all the US labor laws - and with the high prices and other wage issues that would entail, it would drive up cruise prices so high that no cruiser could afford them.
However, the US has a rule about foreign ships. No foreign ship can just hop from location to location in the US without going elsewhere. If they do, they are required to fly a US flag. Of course, since no cruise ship wants to fly a US flag, they make sure they always involve one foreign port. So they might go around the Florida Keys, but they'll stop in the Bahamas.
In Hawaii's case, this is MUCH more difficult. They are out in the middle of the ocean, all alone! So in order to get that "foreign port" requirement, the ship has to steam, full speed, for 2 solid days to get to Fanning Island. This little island is now owned by the US and therefore qualifies as a foreign port. There's really nothing there - a few tourist shops and so on. Its main purpose in life is to allow cruise ships to go around the Hawaii islands without flying a US flag.
NCL has come to a special arrangement for its three Pride ships, that they have a US crew but have special dispensations as regards some rules. In return, they are allowed to sail around JUST the Hawaii islands without making any foreign port stop. So for many people who don't want to "waste" (as they call it) four days just steaming back and forth across open ocean, they go with the NCL ships.
Of course, there are those people who love days at sea where they rest and relax on the ship without worrying about being in a port, so for those people, the Fanning Island destination provides just what they need.