My guess is that with a project of this magnitude (never built this beg before) and the rumors they have had of schedule delays, that they would wait until it is further along an they have some good commitments out of the ship yard (Akers) as to when they could really deliver.
Carnival Breeze - Jan 2013
Carnival Conquest - 2011, Oasis of the Seas B2B - 2011, Carnival Conquest 2010, Carnival Ecstasy - 2010,
Carnival Conquest - 2009, Carnival Ecstasy - 2009, Brilliance of the Seas (Med) - 2008,
Carnival Miracle - 2007, Carnival Conquest - 2006, Rhapsody of the Seas - 2005, Vision of the Seas - 2003,
Rhapsody of the Seas - 2001, Carnival Inspiration - 2000, Carnival Jubilee - 1999