Because the possibility of being turned away at port is a huge deterrent to cruising, and
DCL needs customers, and because it's part of a move away from trying to contain Covid, which is impossible. If you're vaccinated, you're much safer getting Covid, which is why I think they'll require vaccination for years, to reduce the chance of people becoming seriously ill onboard. I said "gradually", because as the islands DCL visits gradually build up natural immunity (aka protection from severe illness, because that's the extent of "immunity" to Covid) due to the massive spread of Omicron, they'll become less militant about keeping people out & reorient towards bringing tourist dollars in.