It's just horrifically out of date.
If you're talking about Linux as in things that are generally applicable to all Linuxes, the Linux Documentation Project (http://tldp.org/) is actually quite well written... but almost everything is uselessly out of date. Most of the articles I've needed in desperate hours of trouble are still written for the 2.4 kernel series. This was especially painful when I was looking into software RAID. There's some great stuff in TLDP, but it's all outdated.
At this point, I think gentoo-wiki and ArchLinux's wiki are some of the most helpful places to go if you're using anything that's not .deb or .rpm based.