In spite of the fact that so many are willing just jump on this as a deliberate, evil money grubbing happening instrumented by deliberate, evil money grubbing executives, I think the reason is much, much simpler.
A ship is a very condensed, confined and close space where all the entertainment, transportation, lodging and dining are in the same place. Managing that type of close knit setup is monumentally easier then one that is spread out over 27,000 acres (or whatever it is now). One on one supervision exists and employees cannot get away at night and are, in many cases, depending on the quality of their performance to supplement their incomes via tips. That translates to the "Cast Member" no matter what their level and because of that, your service level is naturally going to be better.