Many stories that (correctly) get flagged on HN are critically important, far more important than anything which does appear on the front page. There's a simple reason for this: if importance were the high-order bit, then HN would be a current affairs site.
In fact its mandate is something quite different, as everyone here should know—see https://news.ycombinator.com/newsguidelines.html and (for endless elaboration on the point) https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor....
The problem is that there's no counter-action to a group of dedicated users who want to kill a story killing a story.
Moderator review is a counter-action, especially when we turn off the flags on a story. The problem is that we don't (and can't) do that often enough to satisfy everyone.