Because Google made a popular interace to USENET called Google Groups, then added a non-USENET component and discontinued the USENET part. By doing so, a lot of people ended up on the non-USENET part.
A secondary factor was spam getting out of control IIRC.
Usenet still exists!
Yeah but free usenet servers with binaries are rare.
Binaries were the ( second ) death of Usenet. It went from being a social medium to the poster-child of copyright infringement.