Darkness without light is the void. Light without darkness is blinding. They are intertwined, for one cannot see the truth without them. -- Anon Guest
Light and Dark are fundamental elements of reality. Without light, an observer cannot know what darkness is. Without dark, one cannot see at all. Without either, there is no way to tell what else might be lingering about. There's a reason why so many creation mythologies begin with the making of light.
Far, far too many wielders of elemental magics pick only one. Of course, with elements, people pick the oldest and most powerful to battle with. Of course, those who remember the legends think that light is the oldest of elements.
Darkness was always there first. Light can blind, but darkness renders any kind of vision obsolete. Light can burn, but darkness freezes. In a battle between light and dark, darkness has seniority, and thus is ahead by virtue of witnessing every trick.