Caviat: I use a distro with systemd. But these are generally the reasons people don't:

Violates the Unix principle (that Linux has mostly inherited) that every program should do one thing only, and do that thing well.

Subsequent theorizing that the things systemd does could be done more efficiently.

Subsequent theorizing that allowing a single dev team, in this case, Red Hat, a US corporation aimed at enterprise use, to control a large chunk of Linux development could be a bad thing. There is some merit to this, as some applications require systemd specifically as a dependency.

The reality is that these systemd isn't particularly inefficient, and no power moves from Red Hat have been made. Is this the absolute most ideal situation? Perhaps not, and I'm glad people are crafting other solutions. As of now, it does not factor into what distro I choose at all.