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.

It's funny because K5 diaries were one of the very early blog implementations.

K5 was interesting to me, and I actually was the one who negotiated an ad deal between Andover and Rusty to help him with money for the first couple years. K5 had very much the same feel as Everything2 which was the "other" site to Slashdot that the original krewe did. E2 still has a tight community with numerous marriages, people still talking. I think the problem with K5 is that become too self-referential, and ultimately people lost interest. That was the same thing with E2 -- while people for a time loved being part of the community at some point you have to start to do other things in life and the time commitment drops away.