It really depends on the line, as well as the ship in some cases.
Generally speaking, the pay on cruise ships isn't that great compared to some other jobs (although, really, it can work out in other ways, since you don't have to pay anything for room & board, basically you can just bank your paycheck).
Really, it is a lot of hard work. Some people from the States aren't willing to work 7 days a week, sometimes 10 or more hours in a day. But it does have travel benefits, which is why you see a few. Usually you find people from poorer countries working in the restaurants and housekeeping since they get paid more than they would at home, and sometimes the hours are similar (plus they get a month or more vacation between contracts). They're not afraid of the hard work in order to improve their kid's chance at a better life.
You'll probably notice that the management, cruise staff, purser's desk, and shore excursions employ people from more developed countries since they're not gratuity based, and the hours can sometimes be slightly less (and are not as labour-intensive)