two main reasons.
one is ideological. the way systemd was put into the community rubbed a lot of people the wrong way. i won't get into the details, you can google for that whole war. no sense bringing it up again.
two is simplicity. systemd is now over a million lines of code. to put that into perspective, going by the mythical man month numbers, a single good programmer can average understanding 2 pages of code a day in a complex codebase. That's 50 lines of code per day. from the same source, developers produce roughly 10 lines of code per day on average. now, there's a lot of give around these numbers, but you can get some idea of the scale of trying to get a handle on it if there's a bug you need to work with.
a couple of bugs i've had to deal with in recent memory, extended udp handling in resolved and console output (which was actually correct to standard in systemd, but everyone had worked around the previous bug and that workaround wasn't compatible with the systemd implementation). neither of these were minor. the resolved bug prevented adoption of dnssec and the console thing required manual intervention of containers using it (docker, k8s, etc). i don't know if these things have been resolved either in systemd or the container systems. the problems in question forced a rearchitecture of our projects as fixes were not going to be fast enough and we haven't revisited them. for the resolved issue, the systemd project lead flat out said it wasn't a priority. for the container/console issue, you have to go back in time when the docker team wore "no, i will not merge your systemd patches into our codebase" tshirts to conventions.
in conclusion, i use systemd for servers, desktops, and vms. I find it quite reliable in most cases. i think it does a better job with login, hal, service dependency, and mtab than the older system. for my use case of containers, it is entirely unnecessary and nothing but a headache. for my developer station, i mald quite enough and have no patience left to deal with it when it inevitably creates issues (oh, no for this thing you need to put your proxy settings 3 layers of abstraction down over here with this particular format) and tend to use the simplest system possible.