I suppose "elementary" in the sense of "fundamental" or "simple" or "not relying on other skills before you can learn it", rather than in the sense of "easy" or "widespread".

Contrast literacy. Being to read and write one's own language is elementary. It can be grasped by a small child, and has no prerequisites other than vision, reasonable motor control and not having certain specific brain dysfunctions. Yet one does not have to cast one's mind that far back through history to reach the days in which this skill was reserved for an educated minority, and most people managed to live their whole lives without picking it up.