It comes down to where the copyright material is stored. The actual media hosted by torrent users is by the users and as we know over the last 15 years, that backfired entirely. So the easiest way is to take down the tracker.

The files for NZBs are hosted on newsgroups and while obfuscated, is much easier to automate DCMA notices to. Also, the good NZB sites (like private trackers), are tightly controlled so their files are rarely hit vs a lot of ones who have open signups.