I think the biggest issue is that the Hacker News admins want to have as few moving pieces as possible. It's why Hacker News doesn't have collapsable comments, why it doesn't have a mobile layout, etc. There's been some tweaks here and there, but I think the biggest change I've noticed over the past ~3 years I've been here is that they removed karma count from comments, and they made the up-vote triangle high resolution.
They seem feature-adverse, and I assume it is because A) Adding more features lead to more causes of failure, B) Front-end/back-end code additions leads to higher page file size/more computation on the backend (meaning higher costs for them to deliver content) and C) the K.I.S.S Principle
I would have to put my money on the way simpler answer D) A few users might complain, but the traffic keeps growing so why bother and E) YC is eating up all my time and is more interesting and F) The site clearly works (the traffic keeps growing) so it isn't as interesting to hack on anymore.
HN did have a mobile layout for a little while, but it was pretty awful https://news.ycombinator.com/item?id=6253835 and seems to have been discontinued. Maybe they just don't have anybody to make these changes properly.
They seem feature-adverse, and I assume it is because A) Adding more features lead to more causes of failure, B) Front-end/back-end code additions leads to higher page file size/more computation on the backend (meaning higher costs for them to deliver content) and C) the K.I.S.S Principle