Why are 2D vector graphics so much harder than 3D?

A quote from the PDF specification, Chapter 2.1, “Imaging Model”:

By the time the W3C wanted to develop a 2D graphics markup language for the web, Adobe championed the XML-based PGML , which had the PostScript graphics model front and center.Microsoft’s competing format, VML , was based on GDI, which as we know was based on PostScript. Adobe’s PostScript proposed a novel solution to this: an algorithm to “snap” these paths to the coarser grids that printers had. Only Adobe’s PostScript could interpret a Type 1 Font, and only Adobe’s Type 1 Fonts had the custom hinting technology allowing them to be visible at small sizes.

Source: blog.mecheye.net