In the early days of Usenet there were very few ways to access it from the perspective of an average person. You either had to be working on one of the small number of companies that were internet connected or were at a university. That significantly restricted the available pool of people using it, and also filtered that pool.
Up through the early 90s that natural filter mechanism kept the focus of individual groups small, reduced noise, and increased signal. Over time as internet access became more widespread that signal to noise decreased, and most modern forums still have difficulty with it.
There are a few other interesting characteristics - the specific nature of the tree approach and the availability of lots of specific groups gave it some uniqueness - Today you might see a bit of that in Reddit subs, Facebook groups, and other similar platforms, albeit lacking the tree.