You gotta keep in mind that the larger the ship, the less ports it can visit. I'm not just talking about ports of call either. The biggest ships these days I believe only barely clear the Verazzano Bridge in New York. Ships in size will hit a breaking point where land based infrastructure will be unable to handle the crowding, either the shops, bars, beaches, or even the terminal at the ports of call, or the transportation and supplies back at home (I remember the congestion Voyager of the Seas caused on the Jersey Turnpike, and Princess ships cause on the BQE). What's easier for the Port of Miami, parking for 3000-4000, or up to 10000 guests.
Also I imagine many people just not wanting to be on a ship that big and surrounded by that many people.
same