Perhaps it's because they've used the cheaper services and found them lacking? In particular, there seem to be a lot of issues with retention time (years, for Giganews!) and missing posts. Speed is also a factor.
It's not that they haven't tried the cheaper services. It's that those services just don't have the same quality.
There's still dozens of small, basic service providers. Many universities run NNTP servers, as do ISPs.
Cox used to offer a base NNTP service, for example, which was part of my cable subscription. I stopped using it because they had awful data caps and horrible retention.
It's not that they haven't tried the cheaper services. It's that those services just don't have the same quality.