To preserve night vision of the crew on the bridge. It is not just the bridge but the entire bow of the ship is kept dark for the same reason. If you are on a cruise and want to go to the darkest place possible to see the stars in the open ocean go to the open decks at the bow below the bridge.

Cruise ships appear "dark" at night—specifically on the bridge and forward-facing decks—primarily for navigation and safety. The officers on the bridge need to maintain their "night vision" to spot other vessels, buoys, or hazards that might not be visible on radar alone. If the bridge windows were surrounded by bright lights, the glare would make it impossible to see the dark horizon. This is why ships have a "Red Light" policy on the bridge and why guest staterooms located near the front of the ship are often asked to keep their curtains closed or balcony lights off after sunset. Additionally, excessive light pollution can disrupt marine life and interfere with the ship's ability to communicate via light signals if necessary. While the "resort" areas of the ship (pools, theaters, and mid-ship decks) remain brightly lit for guest enjoyment, the exterior navigational areas are kept dim to ensure the ship can be safely guided through the open sea.