Its a historical artifact of how in-line edit emerged as a thing for most people. MIT had huge influence in the development of window systems (X10/X11) and adopted emacs string edit mode in most type in boxes. Bash came from a community which had a strong emacs usage. So, the default in both the window system and in the bash shell, was emacs, even if you use ed/ex/vi

Because most people used awk/grep/sed/ed/ex/vi the regex format tended that way. But enough people used tcsh or bash in emacs by default, that shell and omnibox (when browsers eventuated) edit defaulted to emacs.

Thats my theory: accidental happenstance and defaults.