Frankly, I'm not sure this answers poster's question. While Show HN submissions are visible and volume is high, quality isn't so much. Maybe this explains why Show HNs don't make it to the front (not worth voting up).
I'm suspecting folks aren't submitting to Show HN anymore. They'd rather someone else submit their project for them once it's out of MVP/beta and matures a bit more. It has a much better chance of being noticed and gaining traction then. I also noticed Show HN volume is much higher a day or two after YC-rejection emails are out.
Show HN shouldn't be a platform for advertising startups anyway. Too many submitters try to "show" landing pages which are nothing more than subscription forms, or services which have no published source code and require signing up to even see what it is and how it works, to get viral and get traction.
Being a place for people to show off their code and their (possibly incomplete, and not necessarily for profit) projects is what makes Show HN unique, on a forum otherwise biased towards startups and SV culture.
Of course, I recognize my personal opinion on the matter is not really relevant, but I don't think low volume is necessarily a bad thing.
I'm not against what you're saying w.r.t it being a place to show code and interesting not-for-profit projects, but we can't force Show HN to be that and only that. Forcing an open-source only culture on Show HN is no better than startups/SV forcing a for-profit culture on Show HN. I think we're better off with an open place that's balanced and healthy.
At the very least, though, I think users be able to do something with a Show HN submission besides just sign up for the beta. There should be content worth building a discussion around, even if it's just a mockup or preview of features.
Possibly the quality and traction bit. We've tried "Show HN's" but we've got better traction on BetaList. There's also ProductHunt around as well which may explain the drop in submissions.
I'm suspecting folks aren't submitting to Show HN anymore. They'd rather someone else submit their project for them once it's out of MVP/beta and matures a bit more. It has a much better chance of being noticed and gaining traction then. I also noticed Show HN volume is much higher a day or two after YC-rejection emails are out.