From what I read on the About page of its site, HBSD is the playground of 2 developers focusing on security with the intent of contributing back to FreeBSD. It's hence no wonder it doesn't attract many users.
Furthermore, the BSD family of OS accounts for roughly 1% of the overall OS landscape and, if Linux hadn't adopted systemd in 2014, it would have an even narrower audience.
My feeling is that there is a general awareness of this context in the BSD community and people instinctively stick to well-established BSD OS rather than encouraging further scattering of already scarce and diluted resources. Kind of instinct of preservation.