Why is Hacker News only about "news" and implemented as such?

Some submissions are interesting for a long time (e.g., decades), yet Hacker News and other social news sites select submissions for the front page based on their newsworthiness.

For submissions that are interesting for a long time, there should be a way to show them on the front page for a long time to some fraction of users.

Ranking on the front page would be (partially) based on recent scores for submissions. Users could hide a submission once they are no longer interested in it.

Why don't social news sites do something like that?