Everything moved to the web, and Usenet clients were hit or miss. Neither Windows nor Mac came packaged with a client for it, so it certainly wasn't easily discoverable for people who joined the internet later. They probably never even knew it existed.
ISPs became client-less after broadband became widespread. ISPs didn't want to write or provide software, they just wanted to provide data over basic cable/DSL. Even AOL instant messenger eventually faded, as it never really adapted itself to a non desktop centric web.