Because FreeBSD has really good community support and great documentation, while Debian kFreeBSD is a tiny community. And I really do not see the point - the GNU userspace is mostly available in ports if you want it on freebsd, but the FreeBSD userspace is generally as good.
It's an interesting port, but things don't just work. There's a lot of fiddling around when parts of the system expect gnu utilities that aren't available on FreeBSD, and the BSD utilities work differently. Ifconfig for example does the same job, but in different ways, so stories won't work, etc.
"Finally, the Debian ports to the FreeBSD kernel, kfreebsd-amd64 and kfreebsd-i386, included as technology previews in Debian 6.0 and Debian 7, are not part of this release."[0]
