Back in the 90s, I worked at several ISPs, and let me say that NNTP required more infrastructure than SMTP. First, you need to arrange for NNTP feeds. Then you needed disk space. Tons and tons of disk space, because most of the people using Usenet wanted the alt.binaries.* groups. Skimp out on those, and there was a sizable number of customers who would go elsewhere. Then you needed to configure your expiration policies (posts in alt.binaries.* expire after 24 hours; the rest of Usenet expires in two weeks, that type of stuff).

Then there was the software that supported NNTP---all requiring arcane black arts to keep running smoothly. As much as I loved Usenet back in the day, administrating Usenet (or NNTP) was something I loathed. The server side stuff was just horrible.