In fact, they are.CRUISES SHOULD BE ALLOWED WITH STRICT RESTRICTIONS.
When the no sail restriction was lifted last Fall, the conditional restriction allowed cruise lines to restart... with strict conditions.
They had to put in place all the normal mask and distancing protocols (the same we see at the parks). They had to document rigorous testing of crew and passengers. Then they had to do a trial-run of a cruise with 18+ y.o. volunteers to make sure their protocols were effective.
AFAIK, all the cruises that were affected by the conditional restriction decided to just not cruise at all. If they had done the hard work last Fall, they'd be cruising regularly for the past 4-5 months and into the Summer.