On Linux, `open` was already taken - by a command to run a program on a virtual console (aka virtual terminal; also VC or VT for short). Probably doesn't see much use anymore. I remember in the 1990s, running Slackware, I used virtual consoles all the time. Nowadays, I have an Ubuntu box sitting on my desk, and I use it a lot, but always over SSH or HTTPS from my Mac. It doesn't even have a monitor attached to it any more; back when it did, the monitor would spend months at a time turned off, and on the rare occasions I did use it, I don't remember ever leaving X11 (or maybe it was Wayland, I can't remember). PTYs still get used all the time, but virtual consoles/terminals not that much.
Hence the command had to be named `xdg-open`. Until, more recently, someone decided it was a stupid situation, so `open` was renamed `openvt`, and `open` is now an alias for `xdg-open`.