The Nokton 75mm f/1.5 is not only smaller and lighter than the 75mm Summilux, it's also a sharper lens (with smooth bokeh too). So, it's not just a good lens for the price, it beats the Leica lens which admittedly is an old design.
That lens older than I am

Try a 75 noctilux and you will understand.
Never gonna happen.
The 75mm Noctilux is almost US$14,400. That's $13,500 more than the 75mm Nokton, and it's also three times as heavy (1055 grams / 2.3 lb), which negates the whole idea of using rangefinder lenses and cameras (for me and many others).
It's funny how Leica fanboys (at L-camera-forum.com for instance) always find one reason to say the Leica lens is "better" than the Voigtländer or Zeiss, or at least that they prefer it, but then their priority will vary from case to case. They will say that even though the Zeiss 35mm f/1.4 ZM is optically better, they prefer the Summilux Asph because it is smaller. The same applies regarding the 50mm Apo-Lanthar versus the Apo-Summicron even though the size difference between the Leica and the competition isn't that big. However, they will say they prefer the Leica 75mm Summilux or Noctilux over the 75mm Nokton, even though it is
significantly smaller and lighter than the Leica options. To me, this means it's a brand priority (to say it politely) regardless of optical features, performance, and not to mention the
extreme amount of money they pay extra for the Leica name.
I have one Leica lens left in my bag, the 90mm Apo-Summicron-M, because there are no modern 90mm f/2 M lenses from Voigtländer or Zeiss. The 90 AA is a great performer at long distances, but not so much at closer distances since it doesn't have an FLE. There is also more bokeh fringing than I'd like, so even though the lens is US$ 5,500, it is far from perfect.