"Given a sheet of paper, people write from the top of the page to the bottom. The origin for writing text is at the upper left-hand margin of the page (at least in European languages). However, if you were to ask any decent math student to plot a few points on an X-Y graph, the origin would certainly be at the lower left-hand corner of the graph. Most 2D rendering APIs mimic writers and use a 2D coordinate system where the origin is in the upper left-hand corner of the screen or window (at least by default). On the other hand, 3D rendering APIs adopt the mathematically minded convention and assume a lower left-hand origin for their 3D coordinate systems."
https://www.opengl.org/archives/resources/features/KilgardTechniques/oglpitfall/