Comment by Animats

Right. The S.S. United States was built for winter North Atlantic conditions, with a power plant, hull, and watertight compartments more like a warship than a cruise ship. She could be converted to a troop carrier if necessary, something that had been done to big liners during WWII. Cruising speed 30MPH, top speed 44MPH. Great for getting across the Atlantic in 3 1/2 days. Not so great on fuel economy.

Cruise ships today look like hotels mounted on barges [1]. About half the speed. A refurbished SS United States was never going to compete in that market, and plans to do so were never funded.

[1] https://www.royalcaribbean.com/cruise-ships/icon-of-the-seas