Why are there no good AF 50mm lenses available for Nikon?

ROOOO wrote:

I recently ordered a D600, but am having second thoughts about it after having done a bit more research. My question is simple: why does there not seem to be a great 50mm autofocus lens available for this system? My all-time favorite lens is the Leica 50mm Summicron. The 50mm Summilux-M ASPH is a great lens, though a little too big for my taste. Both of them are super sharp when shot wide open. It seems like 50mm lenses are the best focal length from a performance/cost standpoint. In other words, it seems like it easier to make a great 50mm lens than a great 35mm lens.

Both Canon and Nikon offer good, cheap f/1.8 50mm lenses. Leica's 50mm Summilux and its new APO Summicron are probably two of the best lenses in the world. The Noctilux is the fastest at f/0.95. All of these are incredible lenses. Why is there nothing that's a step or two up from the 50mm f/1.8 for Nikon in terms of rendering and sharpness wide open? (It doesn't seem to me like the f/1.4G really delivers exceptional performance at f/1.4.) Thoughts?