Because regular HD just isn’t enough. The new mod not only jams super high resolution textures into the game but also adjusts other graphics settings (like AA, depth of field etc) to enhance the viewing experience. The creator was also kind enough to leave some recommendations for different CPU/GPU configurations:
The download is still in Beta and comes in at 1.5 gigglebytes (2.34GB after extraction) but really improves the clarity in many areas. Instructions for installation along with mirrors can be found at MaLDoHD below.