It's probably a safety concern. A helicopter hangar requires a very large watertight door. Very large doors are less safe than small doors. In the event of fail

It's probably a safety concern. A helicopter hangar requires a very large watertight door. Very large doors are less safe than small doors. In the event of failure, better to lose the helicopter than the crew. See also: https://en.wikipedia.org/wiki/I-400-class_submarine https://en.wikipedia.org/wiki/I-400-class_submarine