Because torrents have a very low bar to entry. I love piracy! But, I've tried a few times to get into the Usenet scene and I always get annoyed.

1. I have to pay for access to a usenet server. The entire point of piracy is that I'm not paying.

2. I need special, poorly documented, and practically undiscoverable tools to: access Usenet, download a file split over 2million posts, reassemble that file, extract the file from whatever esoteric compression algorithm the uploader decided was best that week.

3. I need to find a Usenet server that carries binaries, then, I need to find the individual groups that have the content I want.

On the other hand, thepiratebay has 99% of everything I want, and what it doesn't have I can usually find pretty quickly though Yandex or sharemania.