It's a feature of the cost-cutting life that is now common throughout the Services. In order to get the new carriers Elizabeth and PoW through on a tight procurement budget, it has been necessary to reduce the headroom between decks to save on materials and weight. This means that Jack will have headroom 5'8" on all decks apart from the hangar.
The Dark Blue has therefore ceased recruiting anyone above that height, as it was further assessed that the costs of protective headgear for people over 5'8", and the possible costs of medical attention when cracking the bonce on the deckhead (ceiling to pongoes/cr@bs) would be too great.