A person can post for 6 months and still they can't be sure about speaking their mind freely , negative karma points accumulate fast and pretty soon they'll be shadowbanned.
Once you are at -15 karma points your replies are not being seen by anybody.
Let's say you are at +60 karma points that you have accumulated in 6 months. It seems a healthy margin, but all it takes is a couple of threads in which you have controversial back and forth with other HN users and all of a sudden you can't speak your mind freely for 6 more months.
Now you have to rebuild back. Only people who post a lot and post popular content earn their right to freely speak their mind on HN.
Most people aren't willing to put up with that.