As for the Panama Canal argument, that's not going to get you very far. The Panama Canal is and will be going to be going through changes over the next few years. They've decided to update to something called New Panamax allowing for your large super tankers, cruise ships, etc to sail through easily. Another thing I've though of as reasoning for such a small beam length is elegance. When you roll on up to Port Canaveral who honestly wants to see a sqaure box hotel on a floating barge?! Not me for sure.
~ FD.
Meyer Werft is in the process of building two ships for NCL that are bigger than the DCL ships they built. I don't know how big the beam is on the new NCL ships that are under construction. Maybe DCL was just going for a certain "look", like the yellow life boats.