It’s already included in the XFCE and Plasma install images, but there’s no good reason as to why it should become the default.
bash is a trusted and reliable shell that has already long proven itself, and that is also used for the initialization of the system. zsh on the other hand is a still fairly new shell that’s popular among newcomers as an interactive shell, but it’s not really suitable as a shell for system initialization scripts.
It is either way trivial for the user to switch their default interactive shell over to zsh if they so wish. 