The content of the article contradicts its title. The tl;dr is that 2D vector graphics uses implicit geometry while 3D is explicitly defined using vertices and

The content of the article contradicts its title. The tl;dr is that 2D vector graphics uses implicit geometry while 3D is explicitly defined using vertices and triangles, implicit means "more maths". And as the article says, the reason we don't use implicit geometry in 3D is that it is simply too hard except for a few specific cases, at least for now. To answer the question "why 2D is harder?" is "because we are not talking about the same thing". From easiest to hardest we have: 2D triangle-based, 3D triangle-based, 2D implicit, 3D implicit.