There are free servers out there, but the majority of them are text only.
Google provides one of the largest [Ranked #10 overall (source, top1000.org)] free services - text only, however.
Here's why:
1. Bandwidth is not free. Storage is not free.
2. Usenet necessitates having a huge amount of both to provide even a sub-standard to average service.
Some numbers: there is an average of ~5.4TB of post data peered daily [source: altopica.com]
The industry leader, GigaNews.com provides 622 days of binary retention - that means they have AT LEAST 3.2Petabytes of local storage, and the same in the AMS/IX farm.
They also provide most large ISPs with outsourced usenet, and their clients as well - so they're easily pushing 50 - 100TB of bandwidth a day [source: 2:30AM maths]
And that's pretty much why.