The LA Times has a great story about the single cruise supplement and how to avoid it.
I can understand that cruise lines count each and every penny.
They know that two people in a cabin spend more than just one person.
Alcohol, specialty restaurants, shore excursions, the casino and most importantly tips means big bucks to the cruise line, hence the single supplement.
Click on the link below for the full story.
http://www.latimes.com/travel/cruises/la-tr-cruise-news-20180211-story.html