https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu...

> The community of emacs editing enthusiasts was adamant that the full emacs editor not be included in IEEE Std 1003.2-1992 because they were concerned that an attempt to standardize this very powerful environment would encourage vendors to ship versions conforming strictly to the standard, but lacking the extensibility required by the community. The author of the original emacs program also expressed his desire to omit the program. Furthermore, there were a number of historical UNIX systems that did not include emacs, or included it without supporting it, but there were very few that did not include and support vi.