Why? Why? Why?
Assuming things turn to ^$%& and an emergency evacuation is required, you don't want to be night blind because of bright cabin lights. If things are relatively dark prior to the disaster, you don't need as much time for your eyes to adjust to the darkness of the cabin and outside of the aircraft.
Although, I've seen some outfits run the cabin lights full bright at night during take off and landing. So, I guess the answer to your question really is: "Because it's our SOP!"
PantLoad