Superior optical performance. The 50mm focal length has been a staple of 35mm photography since at least the 1930s; it boasts some of the best understood and most highly corrected optical designs in history. Even the cheapest 50mm lens will be superior optically to the current crop of consumer zooms. In fact, 50mm lenses are often the sharpest optics in the manufacturer's line. You may not be able to always see the difference in the typical 4x6" minilab print, but you will see a difference (with proper technique) in larger prints.
Makes me want one.