Pretty sure newsgroups were killed by spam and lack of moderation. I spent a lot of time on them back in the day, and by the time Google got in the game they were already dying. By the time they digested Deja, Usenet was a tiny fraction of original size (if you discard spam).
(Not to mention lack of basic features. I've got a laptop at some point and wanted to synchronize read status between my devices. I ended up writing NNTP to Maildir converter so I can use my IMAP server for this. Not pretty.)
