Because cruise lines fill their staff well in advance of their busy tourist seasons.
The summer cruise season in the northern hemisphere, i.e., US and Canada, begins around mid-May and runs through September. Companies are interviewing and hiring many months prior to May. You should have your resume ready as early as November for a job you want the following summer. You could apply in November, start the interview process in December, and get hired by January or February. Of course, the hiring calendar varies from company to company but believe it: hiring for summer jobs starts far in advance!