If I recall correctly based on what I've read about this — I believe from the original mailing list post that noticed the vulnerability — it's because under certain circumstances in order to enable certain functionality you might want sshd to be able to talk to systemd, so distros often patch sshd with code to do that. But obviously you need a library to implement actually speaking system's protocol, and as it happens, the easiest way to do that is to include the entirety of libsystemd, since it has functions for doing that, even though there are at least two other libraries that implement just the communication functionality and are actually designed for non-systemd programs to use. The problem with that idea being that libsystemd, being a whole standard library for all systemd-related functionality that is probably mostly designed for use by programs in the tightly integrated systemd family and as a reference imenentation, also includes a lot of other code, including code that has to deal with compression that depends on liblzma, even though all of that is never used by sshd, because it only uses the small subsection of the library it needs.