> Why not just settle on bash, zsh and fish shell?

When Ubuntu and Debian switched from bash to dash for /bin/sh, the stated motivations were mostly performance (particularly during boot) and size:

https://wiki.ubuntu.com/DashAsBinSh#Why_was_this_change_made...

https://lists.debian.org/debian-release/2007/07/msg00027.htm...

https://lwn.net/Articles/343924/