Because every suggested redesign has been gorgeous as anything, and not as functional or practical. People do this all the time - "Here! Look! I've redesigned HN" - and it's never as practical.
Content over form, every time.
The important thing is the content, not the style.
HN is behind the times, but the Boreal app has made it functional on a smartphone by having a responsive design and some other cool features.
Why do you believe that Hacker News' website looks like shit? What, specifically, do you object to?