The very simple reason for this is that cruise ship infirmaries and medical staff (as well as port hospitals) are not equipped to handle the needs of covid patients who require medical support. The odds of this happening on a cruise ship when people are high risk to begin with is high. It's a recommendation to highlight the risk of cruising if there is even a chance that you might end up needing supportive covid interventions. Among vaccinated people, the ones ending up in the hospitals after breakthrough cases are almost all high risk individuals.