Triangle - Why is it the basic building block for complex shapes
Triangles are always flat, unlike a quad which can be made in a way that is bent / not all vertices on the same plane. 

If you were to interpolate say vertex normals across each scanline, on a 4-edge bent quad it will render with horizontal (scanline) artefacts. If you made that same bent quad out of triangles, it will render correctly as each triangle has a different normal slope. 

That's one reason why triangles are preferable for scanline rasterising