.... Because a rolling release, build-it-yourself based software package model is too big of a hassle (ports tree, I'm looking at you).
I've been a FreeBSD committer for over 10 years. I ran FreeBSD on the desktop for many years, but I switched to running Ubuntu Linux 4 years ago on my desktop because "apt-get install foo" and "apt-get update" are about 10x simpler and faster than doing the same things using the FreeBSD ports tree, and I don't have time to deal with broken dependancies, unfetchable files, etc.
For those who don't know, FreeBSD base system is maintained directly by the FreeBSD "src" team, and is what constitutes a FreeBSD release. This is the "basic" stuff like the kernel, a few shells, fs utilities, ls, cron, a customized system compiler, etc. This stuff is rock solid, and security updates fix bugs.
The "interesting" stuff (X server, web browser, most shells, perl, python, IDEs, etc) are provided by a rolling-release based "ports tree". The big problem is that the FreeBSD ports tree is a "rolling" release. If you need to update your X server due to a bug, you risk breaking some totally unrelated piece of software which has had a version update. Worse, you have to compile all ports yourself when you update, so updates are unnecessarily time consuming and complex.
Compare this to say, an Ubuntu/Debian/Mint or RHEL/Centos/Fedora release where there are no huge surprises when updating. Every apt-get update or (or yum equivalent) fixes bugs, and you don't have to worry about an update to fix program "A" totally hosing program "B"
This is the same reason why Debian or Red Hat based distros are so much more popular than "rolling release" distros like Arch or Gentoo.
I have hope that the new PC-BSD might be as easy to deal with as Linux. I love their "PBI" concept, where every every package contains all of its dependencies. I'm planning to replace an older Fedora on my laptop with PC-BSD 9.