Because it's owned by billion dollar Silicon Valley startup incubator and has a ton of "business logic" for YC baked into it, and it isn't worth anyone's time to bother open sourcing it. Dang has mentioned wanting to do it but it isn't a priority.
You can get the original version of Arc Lisp and the forum it's based on here[0], which only a select few people update, rarely, and they don't take PRs as far as I'm aware. And there is a FOSS fork here[1], which might as well be dead, but as far as what's actually running under HN's hood, good luck getting to it if you aren't a YC employee.
This isn’t slashdot. This is not a server under someone’s desk. This is not a quirky fight for freedom through internet protocols. Y combinator is power, influence, money, and all the abuse that may come with that.
Slashdot, reddit, and HN are similar in that the source code was available. For HN, as part of arc under the Artistic license. All 3 abandoned public source code releases.
My understanding is that for a long time HN was more or less a server under Graham's desk. But I wasn't there so maybe it's more like I believe in a rumor.
You can get the original version of Arc Lisp and the forum it's based on here[0], which only a select few people update, rarely, and they don't take PRs as far as I'm aware. And there is a FOSS fork here[1], which might as well be dead, but as far as what's actually running under HN's hood, good luck getting to it if you aren't a YC employee.
[0]http://arclanguage.org/
[1]https://github.com/arclanguage/anarki