Darkness is arguably the most unique element in existence (though it's also arguably the most abundant thing in existence) and thus one of the most eldritch forces of nature one can warp. This actually makes sense, given you're bending a Primal Fear that is literally nothing more than the absence of light. Whereas darkness as an element is frequently portrayed as acting nigh-identically to its polar opposite, Light, with characters "projecting" darkness as one would light, the shadow element.