1. When it breaks a system it's much harder to unbreak. On a server that needs 99.9999% uptime, this is a potential nightmare.
2. Doesn't support, and the devs of systemd have said they will likely never support... anything other than the Linux kernel. Attempts were made to get systemd to work on other kernels besides Linux and these patches were REJECTED by upstream. So systemd is almost a form of Linux kernel "lock-in". Debian needs to support the kFreeBSD kernel as well and it's a lot simpler to choose something that works on everything than maintain two different standards for the boot process (by default). If systemd could ever work with kernels other than Linux, then it's far more likely to be the default in Debian.
3. The performance gains of systemd quickly evaporate if the system is booted from an SSD.