The UI doesn't have anything to do with it. Hacker News is heavily moderated by smart people, that's why it's a nice place to visit. A poorly moderated HN would turn the site into influence-spamming hellscape like Quora.
Amazon.com is the ugliest e-commerce site going, but they aren't worried about losing visitors to Wayfair.com, because they're not competing on looks.
Same thing with HN, they are not in direct competition with any other site. The high quality of discussions (thanks to moderation) helps the site show up in search engines, which is how most people discover it.
There are lots of alternative front-ends to HN, those are personal projects and not done to try to boost HN traffic, just to improve the UX for regular visitors. For example I use a browser extension that adds additional menu items to the HN orange bar, like "shownew".