[gumby]

EMACS was just a package on top of TECO (the way LaTeX sits on top of TeX). Later re implementations (e.g. GNU emacs) just continued to use the same design.

So why did TECO use a gap buffer? The gap buffer was an easy way to manage an edit buffer back when machine clock speeds were measured in kilohertz and RAM in kilobytes. There were no fonts (no rendering at all), six or seven bit characters, oh, and the machines were often timeshared.

Likewise, vi is just a visual addition to ed, itself a clone of the Multics qed, itself a clone of qed on earlier machines. Those machines were, by modern standards, equally resource starved.