Because they are Disney.
That is the true answer, they pull their cruisers from the pool of Disney lovers, mostly people who have never been on a cruise before but just want to try everything Disney.
They do a wonderful job in entertainment and have nice cabins, but the price is really not justifed unless you have kids and just love everything Disney.
Most experienced cruisers agree that you can have more family fun on Royal Caribbean's mega-ships than on Disney, and at a lower price.
But once again, if you have young kids who love Disney they will be enthralled as the experience involves a lot of interaction with the costumed characters and other kids.
Older kids tend to find it a little too juvenile and regimented, however.