I'll phrase this with a definite "I think".
Remember, RCCL is NOT an American Company, and the ships are registered "Bermuda"?
And I agree with Dave that the ports MAY not be able to handle ships the size if Freedom and Liberty, and the other Voyager class ships due to port depth, ship's draft, and tides. I thought that I have even seen that RCCL ships that leave for the Southern routes even leave San Juan late at night-there has to be a reason.
Then again, that's why I'm good at my job and NOT a cruise line owner. Although I tend to be cynical, I also try to NOT see the "Black Helicopters" in everything that doesn't fly the way I think. If I don't like something, I don't do it again; simple as that.
Hope you have a great time anyhow!