Because nixPKGs have the same Single-Source of Truth wrecking problems as flatpaks and appimages and all that junk.
There's only so much room in the ecosystem for best-practice-violating product, and systemd takes up a lot of that. And until systemd collapses under the weight of doing a thousand things poorly for all the wrong reasons and delivering on none of its brochure features, the other entrants have to wait outside.