I always felt like Usenet was slow, and the idea that you'd have to ask the server for new changes (like email) rather than getting them pushed down to you is what I believe "killed" the platform. Plus, Usenet was distributed so you had to wait until a message propagated to the server you connected on. Discussion platforms built on the Web are (were?) far less difficult to keep together than Usenet, because a company can pay a staff of people to keep the content up and there's only one place to get the content.
Say what you will about anonymity and secure communication, but Usenet died because it was hard to use...not because it was a bad idea.
I never spent any time there, but it warms my heart to hear that alt.callahans is still alive.
My impression of Usenet's death is closely related to your "hard to use" summary: it required people to install and configure a separate client, in an era when many forums were becoming available through the web browser folks already had. (Standalone email clients are far less popular today than they used to be, too. But Usenet didn't remain essential until web interfaces became usable the way that email did.)
