Airlines don't transport people in the same way. A plane flying over the continental US can land in many places fairly quickly if there is a passenger emergency. Cruise ships have a much tougher time with that. If a COVID outbreak was traced to an aircraft, it can be taken out of service and cleaned thoroughly within hours. The same cannot be said for a large cruise ship.
According to CLIA, the majority of Americans have never been on a cruise (somewhere in the range of 75%), so if the 50% are in groups that have never cruised, and have no desire to cruise, that unvaccinated group may not be of concern to cruise lines.