They don't have to fix it or make any improvements. They can run a very poorly performing site with no features and a horrible appearance, because they are entrenched, and they literally own quite a large portion of their userbase.
Unfortunately no matter how many people invent software that does the same thing as Hacker News but works much better and offer it up for free, nothing will be done.
The owner of the site has too much invested in its technology stack (Lisp) and its particular implementation and appearance. The improvements necessary probably require going back on some of those fundamentals, which is never going to happen, because they are too proud.