Any three points define a triangle. This triangle is guaranteed to be planar. You can inspect the winding order of projected points. You can compute a normal.

Go to four points and things are much less simple.

Any four points might define a quadrilateral. But the four points probably do not lie in a plane. The arrangement might be twisted, with no clear winding order.

So when it comes to rendering, that screwiness presents a problem. A set of cases which need special exceptions to cope with. In a high performance system, special cases are bad.

The simplicity of the triangle makes all of that go away.

And should we need to render a quadrilateral, we just draw two triangles. Problem solved.