Usenet died because of 2 reasons: The burden of carrying alt.binaries The burden of pirate media on distributed Usenet servers from alt.binaries
They made a great point of attack for the MPAA and RIAA member associations. And traditionally, ISPs provided these services. I think it was 2006 when Comcast finally pulled the plug. Most other ISPs doesn't even offer a connection at all, which is a shame.
But that's why it died.
The infamous "Green Card Spammers" (http://www.wired.com/1999/04/the-spam-that-started-it-all/) shifted the tone dramatically, and from there it was inevitable that the whole community would collapse. There was very limited control over what people could post, USENET was intended to be free and open. This succeeded wildly in an academic environment where if someone was seriously out of line you could address their university and get their account pulled.
As soon as the general population came on and there was no accountability to anyone it was simply a matter of time before unchecked abuse started to happen.
Most providers pulled the plug because the only thing of any value left on USENET was binaries. The discussions were virtually dead.
USENET is dead because they couldn't keep the troll/spammer population in check. Smaller forums with more control survived because they were more resilient and didn't offer as big a target.