Maybe because it's a pain in the butt to install it? Honestly, I tried FreeBSD 8 two weeks ago, and until there's an easier to way to install it, I can't imagine many people will be interested in it.
If you're looking for a desktop install of FreeBSD, I highly recommend PC-BSD. It's more or less to FreeBSD what Ubuntu is to Debian. It's still a little rough around the edges, but it's a lot more straightforward to get set up than vanilla FreeBSD.
The FreeBSD handbook [1] has an excellent guide on installing FreeBSD from start to finish, as well as explaining all the options of the installer, where to get more help, and how to get the most out of your FreeBSD experience.
