Ignoring the more stupid reasons why people dislike systemd; there's really only three reasons.
The first is just the simple fact that most people don't want to administer their distro as a hobby. Similarly, distro maintainers primarily care about shipping a complete package that they don't need to mess around with too much. Before systemd, every distro had its own bespoke choices in tools and utilities that were wired to work together. Systemd however effectively homogenized all those choices, since almost every major distro settled on systemd. The main difference between distros now is as a result not necessarily the choices the maintainers made, but things like the package manager and the release schedule, so there's less of an incentive to use other distro's. (This isn't some sort of conspiracy, which the dumber arguments against systemd tend to assume; it's just a case where systemd winds up as the easiest choice - systemd has Red Hat backing, wires complicated things together in a way where it works on most novel PC environments that usually require config fiddling when not using systemd and it's just one upstream maintainers have to submit bugs to rather than a ton of different ones. The reasons to pick systemd as opposed to "one million tools" mostly just comes down to systemd being less of a headache for maintainers.)
The second is that systemd violates some assumptions on how Linux software is "traditionally" designed. systemd is a PID 1 process, meaning it's job is to start every other process on the system. In regular Linux software design, this would be the only thing systemd does. Systemd does this, but it also provides a massive suite of services and tools for things that, historically, have been relegated to separate tools. It's a big bulky program, that while it is modular, is essentially competing with a bunch of other Linux utilities in ways that aren't really standardized. This combines with point 1, where distro maintainers near universally settled on systemd, and what happens is that a lot of non-systemd tools that do what systemd used to do aren't really being used anymore even though the systemd implementation isn't necessarily better.
Finally there is a legitimate, albeit niche, case to avoid systemd. Because it's massive and distro maintainers tend to enable a lot of things in systemd, using it means you're getting a lot of random background processes that use up CPU/memory. If you're constrained by that sort of thing, systemd becomes a pretty inefficient hulk of a program you need to tear out.
I do think a lot of the headaches involving systemd would be simplified if the Linux space had any sort of standardization on how to wire it's tooling together, but outside of the POSIX standard (which doesn't really cover this side of things; POSIX is mainly about userspace utilities and APIs, not "how should an OS's system services behave"), there isn't any. People have rose-tinted glasses about wiring together different tiny tools, when the reality is that it was usually a pain in the ass and reliant on config flags, outdated manpages and so on. Just look at the seemingly simple question of "how do I configure DNS on Linux" and the literal 5 different ways in which it can be set since the "standard" proved to be inefficient the moment things get even a little bit more complex than a single network device handling a single connection. (Which sounds like it'd be the case, but may I introduce the concept of wifi?) Systemd being a big program avoids a lot of these issues.