Bash is definitely seen as the "Linux shell", but most users of non-Linux Unixes and Unix-likes don't use Bash. macOS, for instance, now uses Zsh out of the box (although most Bash one-liners still run unmodified in Zsh).
But Linux is the majority of the Unix-like market, so it (and therefore Bash) dominates.
macOS just recently switched to zsh as the default and doesn't change the shell for existing users so I imagine it is likely the majority of macOS users are still using bash.
True, although it gives you a pretty prominent notice that Zsh is now the preferred shell when you start Terminal for the first time after updating to Catalina.
But Linux is the majority of the Unix-like market, so it (and therefore Bash) dominates.