HN was not designed with SEO and linkage in mind -- after a while pages can become "orphaned" after they get pushed down by new stuff. No search engine follows all links. As a practical matter they generally give up after 6-8 links away from the home page.

Actually today Google seems to be indexing most of the site. But usually when I do searches like:

site:news.ycombinator.com alex3917 "rule of thumb"

Only half of those posts show up, and to find the rest I need to use searchyc.com

Personal anecdote, I have no idea if it's 100% related to OP:

http://news.ycombinator.com/item?id=576727

Now google today for "80-19-1"... it's not even on the second page (and it was the 3rd result to me too previously).

I guess OP's point is that Google is "happy" with fresh content, puts it on the front page, then the evil algorithms ;) push it farther.