The fact is that cruising in America is only growing by about 3%/year... that is not enough to fill all the new ships that have some out.
Especially since all of the newer ships are far bigger and more dense in terms of passenger capacity. It used to be that ships were about 80,000-tons and would average about 2200 berths and would max out at just over 2600 passengers total.
The new generation - especially the most recent like Quantum, Breakaway-plus (Norwegian Escape) which is essentially the Breakaway ship with an extra deck for staterooms. The new ships are 160,000-tons and can accommodate over 5000 guests in total.
Now that they have the ships to sell, they need to fill them, and they have a lot of cabins to fill week after week. It also has not done much for the experience of going to Grand Cayman when there are three huge ships there. You can barely walk on the sidewalks to see all the expensive stuff you don't want to buy anyway.