Why has HN not bothered about updating its UX?

Nitpick: I think you update a UI. You would improve a UX. Try reading the words out in full. Also UX is a terribly misunderstood term from what I have seen - I would tend to clarify meaning using different words.

The only thing I'd like to see is a dark theme that follow the system settings. Other than that HN is perfect and loads blazingly fast.

They do update it from time to time. The biggest recent updates I can think of were thread folding and an improvement to mobile styles. Recently there was a thread about adding a dark style, and the mods said they were considering it.

They're just conservative about changing things, and at this point, the look of the site has more or less become part of its brand.

Because it is already perfect!

As far as I can see, there is nothing wrong with its UX.

It works, you can upvote / downvote. It's simple, doesn't need tonnes of JS to run. It's fast.

What do you think needs updating?

The only possible thing I would think about changing is favouriting a post. It's not obvious to me that you click the the x minutes ago part to get to a screen where you can favourite.