You get what you pay for; a simplistic answer, but basically true. The quality of the ship (maintenance level), the training/attitude of the crew, the quality shows, extra staff (kids programs, characters), and the world-class island (
Castaway Cay) are all significant expenses for Disney.
As the PP noted,
DCL also does not have a huge profit center (gambling) and to some extent they lose money on people who bring on alcohol to drink in their rooms. (They still have that hugely overpriced, profitable, and popular photo service, though).
We enjoy cruising on other lines, such as Carnival/RC. Since we drink little, gamble none, and don't pay for photos, we appreciate (really) that the other cruisers who seem to really enjoy spending their money on those things are underwriting our cruises, which are truly much less expensive than DCL. Just look at Vacations to Go today for an example of a $329 Norwegian 7-day Carribean cruise versus $899 for a 7-Day DCL cruise (both plus taxes, etc).
But we always end up coming back to DCL and enjoy it more. We feel better about cruising with them; it feels like home, rather than being a visitor on the other lines. In the non-luxury market, we feel DCL has the best product. And that comes at a cost, one which many (including us) are happy to pay.
You get what you pay for. Decide what you are looking for in a cruise experience; for many DCL is not overpriced since they have a higher quality product.