The linked article covers the major challenges that makes it difficult to adapt the GPU to 2D vector graphics rendering. tl;dr 2D cares about shapes and curves

The linked article covers the major challenges that makes it difficult to adapt the GPU to 2D vector graphics rendering. tl;dr 2D cares about shapes and curves and proper antialiased coverage of super small shapes, the traditional rasterization GPU pipeline is very good at triangles and textures and have limited coverage options. Direct2D, Qt and GTK+ still do a good portion of the graphics work on the CPU, and only use GPU for composition. Some limited graphics can be done on the GPU, usually with quality tradeoffs. Font rasterization is still done the CPU, and uploaded to the GPU as a texture. Newer libraries like Pathfinder, Spinel, piet-gpu all work by not using the triangle rasterization parts but instead treating the GPU as a general-purpose parallel processor with compute shaders.