The monetary barrier to usenet is probably why torrent trackers would be more popular, and thus more likely to be blocked by national regulators. As to why you're far more likely to get an ISP letter for torrenting than for using usenet, while the bittorrent protocol means than if you download a release you're also seeding it to others, paying usenet providers for access to their cached releases means that you're only downloading releases from them, and not uploading anything. Usenet providers do sometimes have to remove releases from their databases upon request, which is why paying for providers on at least two nodes can help in mitigating the odds of a release not being available.

Comparing the cost of a VPN to Real-Debrid + a Usenet Indexer + A Usenet Provider depends on which services you choose, but in the case of ProtonVPN , NZBGeek, and Frugal Usenet it comes to $72 vs $84 a year, with the latter being more if you want to add a backup usenet block plan from a different node (block plans have a one-time upfront cost and last until you use up the plan's download capacity). If you forgo the additional block plan and NZBGeek, instead using a combination of the free tiers from indexers such as Tabula Rasa, NZB Finder, Miatrix, and DrunkenSlug (most allowing for 5 free downloads a day), Real-Debrid + Frugal Usenet is the same as ProtonVPN at $72 a year. Also note that Real-Debrid is able to cache torrents on request as long as someone's currently seeding them on public torrent trackers, and that with usenet to download a release that is X days old, you need a usenet provider with at least X days of retention.

it's not that they sniff traffic for torrents because that's as you said encrypted it's that bad actors can join public trackers and gather all the ips and send them off to your isp if your on private trackers that you trust you don't need a VPN