If the exact rules were available, it would be trivial for spammers to circumvent them by adhering to the letter of the rule and changes would have to be done way slower (because they’d have to be documented every time). It would also lead to lengthy unproductive discussions and lack of flexibility for the sole moderator when applying the spirit of the rule.
You’ll sometimes find posts by dang explaining various policies, and there are some sources which try to document the undocumented:
https://github.com/minimaxir/hacker-news-undocumented?tab=re...
It is my understanding some websites (like dev.to) are automatically killed due to a prevalence of low quality posts. But users with showdead enabled can still see those and vouch for them, which brings them back alive. Posts may also be flagged by users for a variety of reasons such as the subject having been posted and discussed multiple times in the last few days, essentially making the new post a duplicate.