The short version is that Unix has traditionally been built around small tools, well-tested and optimized, that did one thing and did it very well. Systemd is often perceived as kind of a "betrayal" of that idea, as a big system that tries to do everything. A lot of mistrust comes from that criticism.
To add to that, small tools are beneficial to the ecosystem, because they can be replaced as needed. Systemd takes advantage of this to replace existing tools, but cannot itself be easily replaced.
