TOKYO – As an extraordinary two-week quarantine of a cruise ship ends Wednesday in Japan, many scientists say it was a failed experiment: The ship seemed to become an incubator for a new virus instead of an isolation facility meant to prevent the worsening of an outbreak. The viral illness that emerged last year in central China has sickened tens of thousands of people, but the 542 cases confirmed among the ship’s 3,711 original passengers and crew are the most anywhere outside of China. The Diamond Princess cruise ship is also the only place where health officials have seen the disease spread easily among people beyond China. The question is: Why? The Japanese government has repeatedly defended the effectiveness of the quarantine.