I worked in ISP support for years and USENET was dying well before child porn was a nail in it's coffin. Probably has something to do with message boards with much friendlier interfaces, or that ISPs never went out of their way to try to explain what usenet is./p
More probable it was the unwashed masses of those who think that internet == www and if you can't see it in a browser it doesn't exist. Like others, give me a half decent nntp-reader over the most "advanced" webinterface any day.. Guess I'm getting old..;)
I have a theory that it's impossible to prove anything, but I can't prove it.