Perhaps also the comparison with now is shortsighted. Yes, it would take a lot of things to port systemd to a BSD. However, it's not like the features that were developed were developed all at once. The pieces on which systemd relies have been developed over time, and the BSD folks in some cases actively chose not to develop similar or compatible systems. In other cases, it was a more passive decision e.g. deciding to focus developer time in other places. Though of course hindsight is 20/20 and it's hard to predict what features the next cool new thing will rely on, so it is oftentimes also understandable.
But the main message is that the entirety of systemd is not just systemd itself, and the majority of things that make it distinctive and non-portable were developed for Linux and not developed on the BSDs over a large swath of time, not all at once.
It should be noted that similar problems would likely arise from trying to port it to aix, hpux, solaris, OSX, or HURD (I suspect you know a bit more about what that would entail than I ;). But the BSDs are not special in this regard either.
(Incidentally, why the complaint about Linux instead of e.g. OSX's proprietary systems? Why are there not outcries against a lack of e.g. Aqua for BSD? Though perhaps this has been raised and answered elsewhere and I've not seen it.)