It could well be the case that after initiating the evacuation the flight deck crew will be switching off the APU (if it was running) and the engines will have been shut down. The remaining (emergency) lighting will be considerably dimmer than normal and if full lighting had been on up to that point there would be a time lag whilst your eyes adjust to this. By having lights off during darkness this lag is avoided as actually now the emergency lights will appear quite ok or you would be better prepared if things are very bad and no lighting is available