I think people just lost interest, mixture of the community losing interest and the founder losing interest. The rise of blogs didn't help, I don't think: many of the people who in 1999-2004 would've written an article to submit to Kuro5hin would today prefer to post it on their own blog and just submit links to places like HN/Reddit. That has the advantage of letting you "own" your writing, to build a personal brand, perhaps monetize it, etc.; but it does result imo in a bit less of a community aspect.