I gathered that several of the doors were for large cargo loading and storage. If Constellations were designed to serve as deep space galaxy exploration vessels on the frontier, as ProwlAlpha
said, refits and repair wouldn't happen as frequently, nor would supply for the long voyages. They would probably also need a higher capacity for shuttles if the transporter went out for extended periods of time. Doubling up on engines and weapons over the Constitution class configuration heavily implies redundant backup systems in case drydock facilities are not nearby.