A few points,
1) usenet, like so many other technologies, gained in popularity from its use by pedophiles; the most popular groups, abpep-t, ls-series, mclt, etc. carried pictures and videos of young girls
2) usenet waned in popularity when it became increasingly unsafe and pedophiles moved to more secure and more anonymous programs like freenet and TOR onion sites which are used today
3) usenet has also begun to become the target of governments in the West; the first such case in the UK an owner of a usenet subscription service was recently charged with posession and distribution of child pornography for providing the aforementioned groups on his usenet service; there is also the case of Megaupload, which while not a usenet provider, illustrates the trend toward holding internet service providers liable for the content they provide
4) the above case paves the way for governments in the West to charge usenet providers with copyright infringement when pirated material appears on the groups they provide
5) if usenet service providers are made accountable for the content of the groups they provide access to, more and more will cease to operate since most of their business comes from subscribers seeking access to those groups, or they will move offshore; in either case an offshore usenet service is no different than an offshore torrent site
6) too much spam and off-topic material is posted on usenet making it incredibly user-unfriendly when compared to torrents; when using usenet there is often the possibility of downloading illegal content improperly posted in the wrong group and mislabeled
Just my 2 cents