Maybe because the laser light is too intense and can cause vision blur or temporary blindness to anyone who looks at the lighthouse through binoculars. Also, a laser emits light in one direction only, whereas, the electric filament lamp and the low pressure gas discharge lamp glows and emits light in all directions. The laser beam is like one narrow ray of bright light. Laser mechanism is sophisticated when compared to simple conventional set-up. Continuously keeping the high power laser ON will soon damage the tiny electronic components, and this may lead to sudden failure of laser. Low power lasers and special lenses could be designed especially for the lighthouses, but the problem of vision blur and temporary blindness still stands. Gas lasers and solid-state lasers are good for other applications like lidar, optical communication, etc.
