Piracy is more about preserving knowledge than sharing knowledge, after all, torrents are not really accessible for the majority of people. There's a tech barrier plus in most places you want a VPN. Then you need decent bandwidth and a reliable connection. Private Trackers are all about preserving media. Maintaining healthy seeds is important, especially when it comes to media that is very niche or no longer in circulation, "legally". Sites come and go, especially public trackers. Seeders also come and go. Private trackers last a lot longer, and have more dedicated seeders. It's difficult and time consuming to be a dedicated seeder, and it's much more rewarding to host your files on a private tracker than a public one.