Because through torrents you are downloading it through a web of other people's computers. You are limited not only by your own download speed, but by the uploads speeds of everyone you are pulling off of, also at the same time you are uploading bits of the files you have already downloaded for other users. With usenet the file is there on the internet and you can pull it down just about as fast as your download speed can manage.

In other words, torrents are peer-to-peer, which has always been a slow way to go. Somebody with usenet could get SW Revisited in maybe a little over an hour, while you have some people here who have been downloading it off the torrent since the beginning of this week and still are only about halfway done.