Because "normal" implies that Covid-19 is no longer hospitalizing and killing people at high rates. We just very recently (about a month ago) came out of a period of very high death rates caused by Covid-19. If we maintain the current lower levels of deaths, then sure, we can "return to normal". But if the death rate spikes again like it did in the spring? What's normal about that?
Cruise ships are unique. They're often far from medical facilities that can handle severe cases of Covid-19. The countries that they visit don't have the resources to handle more cases. And, as we saw with norovirus, disease spreads very quickly on cruise ships since everyone spends a lot of time in the same places. So, cruise ships took extra care to prevent the spread of Covid-19 last fall and winter.
Then, they gave up. They went to useless 2-day out antigen tests and removed all masking requirements. As a result, cases on the ships are significantly higher than they were before. Thankfully, nearly everyone on the ship is vaccinated and so severe cases are rare. Remove the vaccine requirements and those severe cases will go up, too.