In a response to the question “Why emacs?” on news:comp.emacs, Barry wrote:

At the time I learned Emacs (about 1980), the only other editors on any of the systems I used were line editors (similar to Unix’s “ed”). I don’t think vi had even been invented yet; if it had, it was still very new, and there weren’t many Unix systems on the MIT campus anyway. So if you wanted a visual editor, Emacs was pretty much the only choice.

It has remained my preferred editor because I’ve encountered almost no editors with as many built-in features. And since it’s extensible, just about anything can be added if you need it.