I'm just finishing off a dissertation largely on this topic so let me summarise why.
Bit-torrent sites: Most comply with DMCA take-downs/whatever is an appropriate take down in whichever country. If you remove sites that list any illegal material then you'd have to take down google - which also lists torrents. Selectively banning certain sites is generally a bad idea from a censorship point of view. If only some ISPs did it then people would just move to other ISPs (as has been seen in Denmark (IIRC without checking my dissertation)).
Usenet: much of the content of usenet is very legally - simplified forums etc. In addition, to access usenet you need to use a usenet provider - almost all of which operate a take-down notice procedure and specifically state that it's a breach of terms and conditions to pirate.
In conclusion it's thus near impossible to shut down usenet, whilst its bad policy to shut-down bittorrent. If you look at the bittorrent sites that have been shutdown in most cases (such as elitetorrents) its actually because the owners were uploading material aswell (and thus got done for that) or they got frightened of the RIAA (like torrentspy).