I do not know exactly what happened but one difficulty of controlling a disease on a cruise ship is the incredible population density of cruise ships. I looked at a list of the biggest ships in the world and picked the biggest cruise ship. If that ship was fully loaded with passengers and crew the density looking down from above was in the neighborhood of a million people per square mile. Note this is not the density per square mile of deck, a cruise ship has many decks, sort of like a tall building has many floors. This is simply the density from a birds eye view.
The most densely populated major city in the United States is New York. It has less than 25 thousand people per square mile. So the fully loaded cruise ship in this example has a density about forty times that of New York. Of course density seems to count, as New York is the worst hit major population in the United States.
Perhaps this should provide a little hope that we do not have to repeat the experience on the Diamond Princess throughout the United States and much of the rest of the world.
Furthermore, it suggest part of the reason that cruise ships are so prone to epidemics. The industry needs to step up its game if they wish to recover from this debacle.