Because HN uses a story upvote system, even if you took down off topic "mainstream" news like this over and over, people would keep massively upvoting new instances to the top, because the general thought process is "I wanna go see what HN thinks about trending topic X!" and they upvote the first instance of it they see, and make it otherwise.
Even worse in my opinion is that the poster is incentivized to post quickly encouraging the poster to post the first story covering topics like this rather then taking the time to find a well written and insightful article.
