Cruise ships are foreign flagged because of the Jones Act, not because they want to be. A classic example of a law with unintended consequences.
That is only a part of it.
By being foreign flagged; they don't have to put up with expensive American union workers, and have to worry about strikes etc.
They also avoid many of our taxes; and our litigious society who are constantly suing when things go wrong.
I'm sure they have other reasons. -Tom