There used to be a lot more POSIX OSes; most of them, other than Linux and the BSDs, don’t have Bash, but instead only some other default shell. (Remember the https://en.wikipedia.org/wiki/Microsoft_POSIX_subsystem ? Xenix? Solaris?)
But the modern reason is https://en.wikipedia.org/wiki/Almquist_shell (ash) and its descendant, dash. Busybox environments ship with ash as the only available shell, unless you manually build another shell into the busybox chroot. As well, many initrd/initramfs environments have only one shell, usually dash. When scripting either of these, you have to write pure POSIX scripts.
And—mostly due to inertia—this tends to apply to all such environments, no matter their size. My Synology NAS (because it’s technically a busybox env) doesn’t have bash, despite having huge honking disks! Neither does CoreOS (because it’s technically a PXE initramfs env), despite not being intended as a “link-bandwidth-constrained” PXE environment at all. They’re just descended from these “lineages” of OSes that were used on more constrained devices in more constrained times, so their maintainers have kept to the old embedded-system ways.
On top of this, as the ash wiki article says, Debian made a choice to make dash their /bin/sh, and then forced all system scripts to use it, thus forcing them to be rewritten to be POSIX-compatible. (I think this was done with goal of lower memory-usage for constrained Debian environments that might still need to install some arbitrary subset of Debian system services.)
There was also another kind of pressure, 10-20 years ago, with Linux single-floppy or low-link-bandwidth PXE boot environments wanting to be as slim as possible, and therefore not shipping with bash.
But the modern reason is https://en.wikipedia.org/wiki/Almquist_shell (ash) and its descendant, dash. Busybox environments ship with ash as the only available shell, unless you manually build another shell into the busybox chroot. As well, many initrd/initramfs environments have only one shell, usually dash. When scripting either of these, you have to write pure POSIX scripts.
And—mostly due to inertia—this tends to apply to all such environments, no matter their size. My Synology NAS (because it’s technically a busybox env) doesn’t have bash, despite having huge honking disks! Neither does CoreOS (because it’s technically a PXE initramfs env), despite not being intended as a “link-bandwidth-constrained” PXE environment at all. They’re just descended from these “lineages” of OSes that were used on more constrained devices in more constrained times, so their maintainers have kept to the old embedded-system ways.
On top of this, as the ash wiki article says, Debian made a choice to make dash their /bin/sh, and then forced all system scripts to use it, thus forcing them to be rewritten to be POSIX-compatible. (I think this was done with goal of lower memory-usage for constrained Debian environments that might still need to install some arbitrary subset of Debian system services.)
There was also another kind of pressure, 10-20 years ago, with Linux single-floppy or low-link-bandwidth PXE boot environments wanting to be as slim as possible, and therefore not shipping with bash.