It's undoubtedly because of the population density and the urbanization of these two states. The Netherlands has the highest population density of any state in Europe ( freaks like Monaco excluded). Belgium also has a very high population density. It also has the highest rate of urbanization of any country in the World, so it'll be getting even brighter very quickly.So both states have comparatively little dark open space between vast tracts of lights which apparently fuse into one mass of light when seen from the Moon.