According to that article, the 40 comment penalty is applied to all posts. Putting "Show HN" in your post probably didn't change that.
The algorithm for the front page favours newer things and attempts to generate traction for discussions. I'm speculating here but once something hits 40 comments, not only can it be deemed 'controversial' it could also just mean the article may be able to sustain itself better without more eyeballs, and the front page can 'move on' to other newer things.
Ultimately the Hackernews algorithm should prevent anyone using it as a cheap referral source. 40 comments should have given you some valuable feedback, which is what Hackernews is good for, primarily.
The algorithm for the front page favours newer things and attempts to generate traction for discussions. I'm speculating here but once something hits 40 comments, not only can it be deemed 'controversial' it could also just mean the article may be able to sustain itself better without more eyeballs, and the front page can 'move on' to other newer things.
Ultimately the Hackernews algorithm should prevent anyone using it as a cheap referral source. 40 comments should have given you some valuable feedback, which is what Hackernews is good for, primarily.