Because it does not change the program's UI. It just changes a few editing keys.
I used to know one or two dozen totally different editors in the 1980s, some radically different, such as Acorn's two-cursor system, with a source cursor and a target cursor.
There was no standardisation; this was necessary if you wanted to be able to edit text on any random platform you came across. I also worked with one or two dozen totally different operating systems in those days, from Netware to RISC OS to EPOC16 to CP/M to AIX 2 to OS/2 1 to GEOS to TOS/GEM to AmigaOS.
Now, it's different. In the 1990s there was a huge wave of standardisation. Most of those OSes went away and the few that survived adopted standard UIs.
IBM CUA became the industry-standard UI, for DOS, Windows 2/3/95/NT upwards, for Linux with Qt or Gtk based GUIs, for much of Mac OS X, for OS/2 2.x and upwards.
The same base menu layout. The same keystrokes: Ctrl-S saves, Ctrl-O opens, Ctrl-F finds. Yes even on a Mac (but it's Cmd-S, Cmd-O, Cmd-F, but it's very easy to adjust.)
I no longer have to know a dozen editors. I know one UI and it works on every editor from FreeDOS to Windows 11 to macOS to KDE 5.27 to Xfce, LXDE, LXQt, Cinnamon, MATE.
(OK, not GNOME, but that is just one reason among many that I don't use GNOME.)
It is the industry standard and it has been for a third of a century.
Any new editor I might try must conform to it, or I am not wasting my time.
I don't care how much legacy it has. I don't care how powerful it is. I don't care if it will break 40 years of tradition.
It's meant to be infinitely customisable. So customise it. If the user has any existing config file at all, fine, honour it, turn off all the new stuff and start in 1976 mode. No problem. I have no config. I am 100% happy for that to stay for those who know it.
You don't mention any specific part of this so-called industry-standard UI that CUA mode does not provide. I guess if I were really motivated I could try to figure that out on my own, but it seems like a lot of effort because I'd have to play around with both CUA mode and some industry-standard editor (which I'd have to install first).
* The keystrokes to access the commands in that menu tree.
The terminology. I do not have a meta key. You don't have a meta key, either. No new computer since the 1970s has a meta key. I'm a keyboard collector with a deep fondness for clicky mechanical keyboards; in daily use I use a keyboard from 1991. It predates USB and the "Windows" key. It doesn't have a meta key.
