Well also it creates extra workload for maintainers to handle PRs and issues that come in.
I'd personally rather work on a private codebase, just doing the actual implementation work, and not have to deal with people getting upset that I'm not merging PRs they've sunk a lot of time into.
My guess is that it's very difficult to keep all the details of the secret sauce hidden. They change the details very often. For example the front page is ordered by points/time^1.6, but the 1.6 changes from time to time without notice (I think it was 1.8 for some time, perhaps it's 1.8 or something else now. Some people have analyzed the front page and got compatible results, but I don't remember the exponent they found and I'm too lazy to try).
Dang once mentioned that the actual HN codebase is full of YC business code that would be very difficult to extricate - and that parts of it like the spam filters and flamewar detector they want to keep secret to prevent people from gaming the system. Also that he just doesn't have time to do it.
As far as "secret sauce" goes, the sauce is that the mods actively upweigh, downweigh and filter user accounts all the time. Trying to reverse engineer a working algorithm from this site's behavior is a fool's errand.
I'd personally rather work on a private codebase, just doing the actual implementation work, and not have to deal with people getting upset that I'm not merging PRs they've sunk a lot of time into.