Back in the day, Usenet servers were very difficult to setup. They came with a lot of demands and a lot of machines fell over under the load. Usenet servers also consumed a lot of bandwidth to stay synced. Another problem that crept up is a lot of file systems would fill up prematurely because when the file system was created the block sizes were too large.

In the end, Usenet really taxed the resources of an organization and I think many sys admins just decided all the time and energy required to keep the server up was not worth it.