Perhaps the people who are now famous that posted on Usenet back then didn't want posts from their college days in the spotlight, and their friends at Google implicitly let the site go to hell. </conspiracy-theory>
Edit: Hmm, I may be wrong. A while back, many of the links in this list of "memorable Usenet moments" [1] were broken, but they seem to work now.
Edit #2: Okay, I was not wrong. For example, take a look at the link on that page to "December 1982: First thread about AIDS" [2]. The link takes you to a Usenet post that doesn't even mention AIDS, in the newsgroup fa.telecom.
[1] https://support.google.com/groups/answer/6003482?hl=en
[2] https://groups.google.com/forum/#!msg/fa.telecom/EmQ-s_EGgSA...