Linux (especially Debian-based distros), is usually the first UNIX flavour that people coming from Windows get in touch with. While FreeBSD has excellent documentation, Ubuntu and others simply have the larger newbie-friendly community.
This sums up my experience. I still have an ISO of FreeBSD 10.1 sitting somewhere on my system just waiting for me to copy over to a USB drive once I work out how to set it up on a dual-boot system along with Windows.
So far, all the guides I've seen for dual-booting have been for Linux, and they've all been straightforward.
Linux (especially Debian-based distros), is usually the first UNIX flavour that people coming from Windows get in touch with. While FreeBSD has excellent documentation, Ubuntu and others simply have the larger newbie-friendly community.