Firstly, because this site happily handles the amount of traffic that puts many hobbyist sites that happen to get on its front page into a hug of death; so its developers must have done something right on the backend that is probably above the web programming 101 level.
But secondly, because this was precisely my point. One does not need a super popular front-end framework to make an awesome web product, and the HackerNews site is a testament to that.