The ZSH shell would be actually great choice for shell in the FreeBSD Base System because it uses the MIT license.
As BASH uses the GPL3 license it does not have the chance of being included in the FreeBSD Base System because the FreeBSD project currently tries to remove all GPL3 licensed software from its base.
And for good reason. If people already call command completion bloat, imagine the uproar if they found out that bash or zsh is built in and the default shell.
FreeBSD is often used as the base for appliances, so some downstream users it may indeed be unnecessary code to maintain. (You perhaps get 'extra' CVEs against your product that may increase maintenance.)
For those with CLI access, adding a more full-featured shell is fairly easy with pkg.
Keeping more complicated stuff in ports rather than base means users can easily get the exact version they wany so FreeBSD tries to keep base as explicitly a base system.
It's a tradeoff but one I find more useful than not.
As BASH uses the GPL3 license it does not have the chance of being included in the FreeBSD Base System because the FreeBSD project currently tries to remove all GPL3 licensed software from its base.
Details here:
https://wiki.freebsd.org/GPLinBase