One advantage of vi is that even though emacs is available on all those systems, vi is actually installed on all of those, setting aside windows. If you know rudimentary vi you can walk up to any of those machines and edit a configuration file well enough to work.

It's quite difficult to find software older than Emacs in widespread use. Emacs is one of the "original" software, the first GNU software for which the GPL was created. It competes with vi, whose direct lineage has been broken (nvi and vim are reimplementations).