Because journalists have an incentive to attract readers, and it worked. The blog post made it to the front page of HN.

(I don't think I've ever written a news article about a tweet and likely would not, barring extreme examples such as @WhiteHouse tweeting that the president had resigned or somesuch.)

Blog post about tweet going "massively viral" leads to chorus of outrage on popular Internet boards (news.cnet.com)

Slow news day?

Or a Hacker News link to a blog post about a tweet. The flag button under the headline isn't there for decoration.