The reason we have heard is two-fold: cost and craftmanship.
1) Ships made in the US are required to use all US personnel to build them, many of them unionized dock-workers - not cost effective.
2) It is hard to find the companies experienced in working with all the materials found aboard modern cruiseships in the US. Much of the work requires trues artisans.
The only new cruise ships being built in the US are going to the ACV, American Classic Voyages, and they are about 1/2 the size of Destiny but cost about the same. They are building them here because they want to sail between all US ports and according to the Jones act those are the only ships allowed to do so.