You are thinking about HN as if it was just a piece of software. It isn't just a piece of software; it's also:

* an important piece of YC's infrastructure, so it doesn't have a normal dev team; some of the data going through HN is "hazmat".

* a demonstration of a new programming language Graham is ostensibly working on, which implies that doing things "the right way" is more important than on a typical project.

* in part a secret (the published HN code is not the code running on HN).